Cavitating Ventilating Prop pictures added

[BilgeBoy]

Hi all

A while ago I posted a question regarding my prop ventilating (thanks Kerry). I have had a chance to add some pictures that show the height of the engine & prop. I was unsure if the minor prop damage I have is the cause of the cavitation orif its the way the engine is set.

The thing is OK out of the hole....but as soon as you turn (Mild Turns) revs come up and boat slows down...

Here are the pictures

[BilgeBoy]

Straight edge to keel line...

[BilgeBoy]

Prop Damage...

[blaze]

The idea of the foil is to skin on the top of the water (I dont like them), I would think to have any chance of that happening with what I am looking it needs to lift at least an inch, dont know if being so low its causing a tubulance around the prop.
cheers
blaze
ps have you tried with the foil off

[hussy]

motor to me looks like it is too far in at the bottom, and i would still go down another 25ml and tryit if you can. hubby

[Reef_fisher]

have the same problem, good going straight, turn and prop cavitates easily. I have gull wing hydrofoils (came with boat) and wondered if the extra lift was lifting too much. have aluminium boat 4.5m and 50 2stroke. Just thought it was a fishing boat and not a speed boat. thought a anticav plate would be better but don't have the dollars to try. Be interesting to see the replies to this thread.

[fishingjew]

Had a bit of the same problem on my last half cab with no hydrofoil turned out to be prop. Had worn about 10 mm off blade tips over the years . Take it to a prop place and have them check it and the pitch. A new one solved my problem and yours looks a bit worn


# #Russell

[youngfisho]

mate that prop looks pretty tired. Maybe a new one that is cupped may solve the problems you are having. As others have suggested try taking the foil off and see how that improves the performance of the boat. How fast are you travelling when you are turning and what trim postition is it in.


andrew

[finga64]

Gidday Bilge boy,
Mate, you say your boat has excellent 'out of the hole' performance.
How does the boat ride after you've been on the plane for awhile going in a straight line?? Does the boat sit nice and flat or does the bow aim for the sun??
Have you tried to adjust the trim more then a hole in to see if that makes a bigger difference??
Is the straight edge in line with the keel (ie hard up against the bottom of the hull) or is it just showing a line from the cav. plate to the keel line??
Sorry for the questions but sometimes the seemingly obvious can be accidentily overlooked #:)
A picture side on but further away so we can see the overall picture (say back 1/2 of your boat) would probably help as well.
Good luck with the problem matey. They a pain in the a-- sometimes to figure out.
At least it's a good excuse to get on the water....to test the boat ;)

[gawby]

To me your prop looks like it is a hundred years old and your motor 12 months old.
By the looks of the straight edge in the pic it is facing up from the keel and i think you should drop your motor down a hole and that would bring the cav plate in line with the keeel.
When i bought my first boat i would have trouble with it cavitating and a skippa came with me one day and gave me some tips on driving a boat and one of them was to trim the motor in when turning hard. Doing this stoped all me problems.
Graeme ;)

[seatime]

Hi

all good advise above, first thing - lose the foil and see how it goes for out of the hole, and you'll get a better idea of trim angle (motor leg angle to transom).
second - get a new propellor, that one, will in time cause damage to your donk gearbox and top end , get it fixed and keep as a spare.

my guess, the foil causes the ventilation on turns, they're meant to assist in initial out-of-hole planing.

regards

[Kerry]

Need to see a side-on view of the last third of the hull/keel in relation to the line of the cav plate.

That foil needs to come off # :-X and see what affect this has.

Those pressed "ripples" in the hull bottom could also create a water fall/air affect in a turn, fine in a straight line but in a turn water/air could be going every which way? If so nothing you can do with these?

I wouldn't be dropping it down anymore, appears low enough (from what can be seen)?

Regards, Kerry.

[BilgeBoy]

Hi all

Cant thank you enough for the posts...this is driving me nuts!! Ok will rip the foil off and try without first and will get the missus a new prop fpr christmas ::)

I have an electric trim and have tried in and out on turns....doesn't make a huge difference. Gets up on the plane pretty quick (imaginge that,s the foil working) and once up is fully adjustable...Can bury the bow until it's all wash and rpms drop...or can trim up until bow high and boat all unstable. Speed when turning....must admit it only occurs on higher speed turns...say 15-20 Mph.

Finga...yeah mate...the ruller is level with the keel. I can come in a little on the trim from where the picture was taken. Have attached another picture that shows side view engine was trimmed in as far as it would go. (will get a picture of back 1/3 of boat also).

Thanks again for the posts!!! :)

[NAGG]

For me .... it looks like the motor needs to be dropped a hole >>>>> also I dont know if its just me ...... but are the foils mounted in the correct position (shouldn't they be mounted on the cav plate???????
NAGG

[NAGG]

Sorry .... I didn't see all the pics (re mounting of foils) :'( :'( :'(
NAGG

[Loco_Pez]

Motor height looks OK to me. The prop wouldn't be helping by the look of it.

LP

[Sniper]

Do recall a topic similar to this ages ago about the difference between cavitation and ventilation. May help to understand what is causing the problem. Hope you get it sorted. Nothing more frustrating.
Cheers
Billyboy

[Stumpy-USA2]

To expand on what Sniper said.........
Cavitation is air bubbles around the around the prop. that cause a loss in thrust. It is caused by inefficient or damaged props. The damage to yours is most likely causing some cavitation.
Ventilation is when the prop. breaks loose due to air being sucked into the prop and causing a loss of thrusting power. This is most often caused by props. breaking the waters surface or a prop. that is too small and over powered by the motor. If your engine is too high, or turbulence is being introduced into the props. working area you could be having problems with ventilation.

My suggestions would be to first loose the wing. If that does not clear it up, drop the motor slightly. As a last resort, go to a cleaner prop.

The "ripples" Kerry is referring to are the chines. They are made into the hull to give lift. More lift means less drag and more speed. As a boats speed increases it will lift out of the water and the chines act as new running planes. The higher a boat gets the more efficient the motor becomes by gaining rpm due to the decrease in hull drag. The chines are in a stair case setup, really fast boats will lift high enough that the very lowest set of chines are the running surface. On most bass boats over here only about 18 to 24 inches of the lowest/narrowest set of chines is in the water when running on plane. Its what we call being on the "pad." As a rule of thumb we run the largest pitch prop. that our motor can turn to full rpm to get us to that point in performance. The chines are also what makes a boat turn at high speed. They act like mini keels that dig into the waters surface like the edges on a ski allowing the boat to turn instead of skipping or sliding across the water.

Sorry if it sound a little preachy. I had the same problems with my boat. had to change props. and raise the motor up to solve it.

Casey

If there is anything else I might can help with just send me a message.

[finga64]

IMO by the look of that the motor has to come up about 50mm.
You want the cav. plate about 12-20mm above the lowest point of your hull (not the chunk of alloy that makes up your keel though #:))
If you draw a line across the very bottom of your two bung holes and draw a line 12 mm up from that that would be pretty close to where you want your cav. plate to be level with.
I see you have a spare hole to raise the motor. Try that last hole first before drilling some new ones ;)

I really don't think the props that bad. I seen a lot worse on a lot of tinnies and they don't play up. A trip to the prop doctor may be on the cards though just to check that nothings bent. If you want it all spiffy then they should be able to repair as well. There's a good one down that Gold Coast somewhere #:)
Cheers Scott #:)

[seatime]

finga

are you sure the motor needs to be raised, it looks OK from the photo.
but then it's difficult without seeing it in the flesh, or test driving.

trimming out would lower the cav plate slightly before it started to come up again to achieve optimum trim. I always thought the cav plate should in the water when planing, so water is held down on the prop.
but then I'm learning everyday, and getting it wrong sometimes too :)

regards

[finga64]

finga

are you sure the motor needs to be raised, it looks OK from the photo.
but then it's difficult without seeing it in the flesh, or test driving.

trimming out would lower the cav plate slightly before it started to come up again to achieve optimum trim. I always thought the cav plate should in the water when planing, so water is held down on the prop.
but then I'm learning everyday, and getting it wrong sometimes too #:)

regards
The cav. plate should be on top of the water or just slightly above. :)
There would be tons of spray going everywhere and lots of drag if the cav. plate or if you put a set of foils on and they live below the waterline. :)

[seatime]

okey dokey, thanks :)

[FNQCairns]

Certainly low enough, as said above that prop looks cactus (make a good get you home prop as a spare), if you change it you will probably be very surprised, I suspect the engine is overpowering it and causing a plughole effect, lots of tinnys with thier small V and high riding rears on the water cannot go up real far. Going down will stop the symptom but make the boat a bit of a dog in every other respect.

I would try and fix the prob without the foil.

cheers fnq

[finga64]

this may help #:)

Example #1 - Boat with 12-13 degree transom angle (typical of most boats) provides good bow lift when outboard is trimmed out and a top speed in the low 50 MPH (80 km/h) range. With this combination, the anti-ventilation plate runs parallel to the water line and can be mounted 1 in. (25 mm) above the boat bottom.

[seatime]

won't that create more ventilation, and hence cavitation around the prop blades if the prop is breaking the water line?

how does the trim tab under the cavitation/anti-cavitation/anti-ventilation plate work if it's clear of the water?
it'd have no effect on counteracting thrust, wouldn't it?

sorry finga, those diagrams seem to be out of context, and only confuse me.

[BilgeBoy]

Hi all

again thanks for the posts. I have one other question. the prop is a 10 3/8 with a pitch of 13 yet every web site I look at regarding props for 50Hp yamaha are quoting 12 x 19 ish?

I do remeber the guy I bought the boat off saying that he got this engine off a mate (who had upgraded to a bigger engine) so perhaps my prop is way off :-/ I will try this prop guy on the coast etc....

Can I also assume that the Cav plate is the one that the foil is bolted to????

Megsy

[finga64]

Gelsec....The trim tab is still in the water. The cav. plate is level with or just slightly (1or2mm) above.

The foil should create lift for the motor to get the cav. plate level with the waterline quicker. Thusly, and theoretically, get the boat planing quicker and at lower speeds. Just like the foils on the Hydrofoil ferries in Sydney Harbour.

The foils are usually angled upwards and outwards so there's less drag as the boat goes quicker. ie there's less foil in contact with the water

Yeah, the picture is a bit howdydoody. Sorry about that #:-[

Try this link it's a lot clearer to see what's happening
http://www.themarinedoctor.com/Tillermodels.html #:)

Bilgeboy, according to Solas site the pitch is close, not sure about Dia byt the second site says your prop is close to the mark :)
http://www.solas.com.au/shopexd.asp?id=120 or
http://www.ptprop.com/catalog/default.php?cPath=114 and select the oppropiate engine (it's already in the Yammi section #:)

Some general info about props
http://www.boatfix.com/how/props.html

[Chimo]

Hi Bilge Boy

On four boats that I have had fins on I have tried and eventually succeeded in in getting them to work as they should by adjusting motor height very much along the lines shown in the diagram posted by Finga. #It seems that if the motor is at the correct height you get benefit from the fin at lower speeds due to the lift resulting from water flow over the top of the fin; much like the lift on aircraft wings due to the vaccuum #on top of the wing. #Thats why a foil on a boat has a flat on the bottom(short distance) and a curved top (longer flow path / greater distance therfore a vaccuum therefore lift) on the top.

After your up an going you dont want the fins to drag and slow you down as the work has more or less been done so when you trim the motor up the fins would ideally be pretty well above the water surface. #I can pretty well put my fingers below the fin when the boat is running at planing speed.

For all this to be able to happen you need a decent propeller.

One that ideally works even in the surface piercing mode. #For example if you look at stainless which I believe is the only way to go, and look at a couple of dfferent props they do have different features and different characterisics. #In my case if I run SS Vipers instead of SS Rakers I have to drop 1'' of pitch as the Viper likes to be deeper into the water where as the Raker is happy to run in a surface piercing mode and so the motor (s) can come up to where they need to be to get all the performance that the boat and motor can deliver.

From what you have posted it looks like the motor could go up and you do need to have a good look at that prop. #Apart from being Al and very tired is it the correct one for the boat?

Can you deal with a SS prop supplier and try a few SS props of sizes perhaps more suitable. #Solas is claimed to be pretty good

Any way hope thats of some help, good luck with it

Cheers

Chimo

[FNQCairns]

That's your dia n pitch!!!!!! I was going to ask your WOT revs in my post but it's hard to get a reliable answer sometimes.

You have our problem - a tiny prop- it's simply being overpowered, the ventilation plates (cav plate) job finishes when on the plane to most degrees as the water past the prop is fast unless as you are -way out of bounds :o. Just re prop it!!! and enjoy

cheers fnq

[Loco_Pez]

Try this link it's a lot clearer to see what's happening
http://www.themarinedoctor.com/Tillermodels.html #:)http://www.boatfix.com/how/props.html

I note that on the web site to which finga refers it says that the cav plate should be "within 1" of the bottom of the hull, not exactly 1" from the bottom of the hull, so if it is level with the hull then that would still be OK. #Yes ? #If so then from the photos it looks like the height might be OK in which case I think it is more likely the prop may be too small.

What WOT RPM are you getting ? #If it is top end or above recommended for the motor that would also suggest too small a prop.

LP

[Spaniard_King]

Fix prop or renew + lift engine 1 hole = a lot better performance(for this boat)

Garry

[seatime]

Yep, I've now come around to the idea of lifting 1 hole as well as a new prop, first thought height looked OK, recent experience has changed my mind.

When water testing a new boat, which hadn't been out before, we found the motor was too low. There was lots of spray and it needed a ridiculous amount of trim to correct. The anti-cavitation plate was already 20mm or so above the lowest point of the vee, and obviously needed to come up another hole.
While this sounds like it will be quite high, the conclusion I made concerned the degree of deadrise was, the deeper the vee - the higher the prop needs to be. This particular boat had a 18-19 deg vee and was alloy 6m LOA.
Compared to my 4m pressed tinnie, which has bugger all vee, the anti cav/cav plate works well in line with the bottom of the hull.
There doesn't seem to be a hard and fast rule regarding motor height, it's more a boat by boat specific set-up, trial and error till you get the happy medium.
The deeper vee boats never really get right out of the water, they're still displacing to a certain extent, so, the prop can sit higher, whereas flatter pressed tinnies with relatively bigger strakes are more up on the surface of the water, and the prop can't be too high, especially for turning.

Well, that has been my personal experience anyway. They can be fiddly sometimes, though when you get it right it'll be smooth sailing from then on.
Good luck

Steve.

[Kerry]

Gelsec, Deep-V, No-V, tiwns, single the principle does not really change, cav plate between level to an inch below the bottom of the hull directly in-line with the legs.

With pods then you can start lifting the cav plate as a general rule of thumb of 1" per foot.

Pressed tinnies generally only have clean underhull lines in a straight line. Some of the tinnie keel designs are the worst offenders.

Regards, Kerry.

[griz066]

Following this post with keen interest but wwwwhhhhooooooooaaaa back there a bit. Now we have Kerry & Finga sayin the opposite, so which is it cav plate up to 1" above the bottom of boat or up to 1" below ?????????????????????????????????????????????????? ???

[Roughasguts]

Think I would just put the right size prop on it.
The one on it looks way to small.

[Kerry]

Standard normal recommended fitting height is generally between the bottom of the hull and 1" below and from there it can be trial and error depending on the setup.

Generally if the motor is fitted properly you should be able to drop/raise 1 hole and preferably be able to raise 2 holes.

[seatime]

They are both right griz066, I wasn't factoring in the pod enough, as it sits a couple of inches above the vee anyway. Sighting by eye, the prop and cav plate will sit a lot higher than I expected.

Still figure it's a boat by boat, weight distribution, fine tuning principle tho. Two boats of the same hull with different motors, different weights onboard (fit out) and cav plates at same height, may not always perform the same, and a bit of fine tuning may be required, an 1" up or down to get desired result. Pods are certainly different.

Comparing the 2 boats in my earlier post, I got some first hand experience of how they performed, sitting in the back with tiller steer was a big difference to a podded plate boat set-up.

I don't think the textbook suggestions can always be followed to the letter, some fine tuning might be needed for individual boats.

We are all on the same page however. Point taken tho Kerry,ta.

regards

[finga64]

I've shown some reputable sites showing from level up to 1" above the bottom so I'd love to see some information from an installation guide, manual or website using the 1" below figure. Can't say I've ever seen information saying to mount 1" below but my views may be be altered if I'm shown some new information and I think it's credible. # :)
Yep, found some saying below so I've learnt something new today ie Honda and Tohatsu owners manuals :)
So start from level and work your way up or down sounds the go.
The critical thing is apparently where the waterline of the boat is.
I personally have found about level to an inch above works for me though.

[Alex9797]

Hey BB :),

Mate I had a similar problem on a 15 foot tinnie runabout. It mainly broke loose turning one way more than the other, can't remember which way though.
Is your boat a runabout , tiller , centre console?

I don't know a great deal about things mechanical. However what I did learn with mine after consulting with a panel of experts and reading the inards of several chickens........... I was a fat bastard and having 115kg at the front of the boat tended to break the prop free when I turned in one direction ;D ;D .......which mean't that I had to remember to trim in on hard turns.

Now I'm not suggesting that you are anything other than a lean mean fighting machine, but as as a famous treker once said"ya kanna beat the laws of physics captain".

Disclaimer:

I am only relating my experiance with my particular hull and gut so will not be held responsible for any replies from Kopey or Kerry relating to my obvious lack of technical knowlage :)


cheers

Alex

[BilgeBoy]

Hey Alex9797

Never though of adjusting my double chin ;D ;D ;D We must be twins mate...I am about 110Kg so you just may be onto something. And yes it is a 4.35 Front steer.

Have tried filling both tanks, esky, and putting 2 kids as far back as I can (don't want to loose them...they have cost me too much in food) all in an attempt to try and balance the boat a little better!

Everyone else....Again thanks for the posts!! I havn't been able to get it wet since I posted ....and this weekend doesn't look good. But as soon as I can get out...I will try trim in and out on turns...I will try taking the foil off...Plenty of sandbanks in the bay to make adjustments on ;) And I will give some of the suggested prop places a call and see what they recommend.

Will keep you posted...

Megsy

[finga64]

How did you get on Bilgeboy??
Problem solvered yet??
:)

[BaysideMarine]

1) It is quite common for aluminium hulls to cavitate in turns usually due to the cav plate not being in line with the lower edge of the keel extrusion. If the cav plate is level with the bottom of the hull (but not the bottom of the keel extrusion) you will have problems

2) Ditch the stock prop and buy a decent prop. Your brand of engine is known for its poor props. A quality aftermarket prop will do wonders and (as previously mentioned) a cupped prop will aid in combatting turbulent water (thats what they are designed for)

3) Your engine height appears fine

4) Put your foil out for the next rubbish collection. Foils are used as a patch up job for an underpowered rig, to aid in getting on the plane in the abscence of appropriate power. But more importantly, foils generate LIFT in an area that is lift/drag nuetral so in a turn foils tend to roll boats over and in severe cases will roll a boat onto its side due to the lift being generated that shouldn't be.


Cheers,

Nicholas

[Kerry]

1) It is quite common for aluminium hulls to cavitate in turns usually due to the cav plate not being in line with the lower edge of the keel extrusion. If the cav plate is level with the bottom of the hull (but not the bottom of the keel extrusion) you will have problems

Not so! The bottom of the keel extrusion is not the relevant point.


4) Put your foil out for the next rubbish collection. Foils are used as a patch up job for an underpowered rig, to aid in getting on the plane in the abscence of appropriate power. But more importantly, foils generate LIFT in an area that is lift/drag nuetral so in a turn foils tend to roll boats over and in severe cases will roll a boat onto its side due to the lift being generated that shouldn't be.

;) Now that's something I can agree with!

[BaysideMarine]

Its ok if we don't agree Kerry :)

I know that I fix a lot of tinny cavitation issues, actually its probably ventilation really due to air flow along the keel extrusion that gets drawn into the prop flow, by making sure the cav plate is level with the bottom of the extrusion. The point where the extrusion tapers off towards the stern is important also.

But I am done arguing on forums. I know what works for me when fixing these problems and thats all that matters.

Glad we agree on the foils :)

Cheers

[Kerry]

For sure the keel extrusions are a real concern but just because a keel extrusion is 75mm deep and 20mm wide does not impact on the general flow of water towards the motor, not in a straight line anyway.

In a turn then that is entirely a different matter but this is simply a design problem and not something requiring the motor to be dropped 4' as that simply creates more problems than one wants to know about.

Similar issue with centre mounted singles on Hydrofields, one could use a 25" motor to get below the air but it really did not serve any usefull purpose except make the engine sit in the middle of the transom.

[BilgeBoy]

Hi all

Quick update on where we are at....

Finally got a chance to have a play on the weekend and took a run over to Coochie. One the run over I got the missus to steer while I stuck my head over the back...the first thing I noticed was all the spray going everywhere....what a mess. Looked to be coming from the sacrificial anode area just above the cave plate and at 20Mph the foil is about 3/4 inch below the water line. Tried to trim in on turns and this was marginally better!!

So park her up on th beach...and whip the foil off. Load the crew on so we have the same conditions and go for a squirt....What a pig. The nose heads for the sky and is real slow to come back down. Onto the electric trim and in as far as I can go....the nose slowly comes down....but not as far down as with the foil. In fact with the foil attached...I can push the nose in until I get wash from up past me (front steer). Without the foil...its all nose in the air and unstable in the rough stuff.

I haven't done anything with the prop as yet....Will soon enough (Saving for a sounder at the moment). In the meantime I will RAISE the engine a whole...and see how that goes with and without the foil.

Hey I figure people have done a lot of 2 fingered typing on this post....so the least I can do is be a Guinea pig, try different suggestions...and report back!! :)

BilgeBoy

[Homer_Jay]

Not real sure if any of this info will help but here goes.....

About ten years ago my old man bought a 4.5 mt tinny runnabout brand new with a 50 Yammy on her. When we got it home and took it for a run it would vcavitate the prop all the time even in a straight line getting onto the plane. Needless to say he was not happy so he took it back to get it fixed. After alot of arguing and trouble they finally agreed that it wasnt right and they couldnt fix it so then Alf Stessl got involved and in the end they got it to be allright but it would still cavitate in the turns at any speed. What they did was put on a cupped prop. The old man just put up with it. Then about 18 months ago he decided it was time for a new outboard. Being pretty happy with the Yammy he got another one but this time got a 60 hp. The first thing we noticed was that the prop was so much bigger in diameter from the one that was on the 50. We were wondering if we were going to have more problems and have to get the prop cupped. But we took it for a run and what a different boat. Now the boat wont even cavitate in the tightest of turns. He couldnt believe it was the same boat. All that we can put it down to is the bigger prop. So dont discard the prop issue too easily. I would go have a talk to a prop expert.

[Chimo]

Hi Bilge Boy

Just looked at the photos and read you last comment and it really looks to me that the motor is too low.

Added to that impression is your comment that the foil is 3/4 " below the water when your running at speed on the plane which also reinforces this opinion.

My experience with cavitation foils etc on many boats is that if motor height is satisfactory and foils are fitted #
1 #there is not too much water /spray being thrown around and
2 #I have always been able to stick my fingers / hand between the top of the water surface and the foil when planing at speed

There is too much drag if the foil / motor is too low. #

You want the foil to do its thing as you go from displacement speed to planning speed at which time, if you don't have trim tabs you would be trimming the motor up, lifting the foils even higher, and making your steering lighter.

The prop could also be part of the problem too.

Perhaps you need to lift the motor: its too low anyway and your observation re the foil makes that plain; and talk to a good prop person and or maybe borrow a few to try and see the difference props may make.

Good luck, you'll get there in the end.

Cheers

Chimo

[a_big_red_1]

B/B reading your last post, IMHO i would lift the motor 2 holes
i have just been going through all this with my boat
lifted 1 hole made a difference
lifted another hole and it is a completely different boat

Daz

[BilgeBoy]

Found this little Gem on the Solas Props Web site....basically a warning at the top of their stainless prop page...looks like I will be raising the engine a whole before buying a new prop!!

http://www.solas.com.au/shopdisplayproducts.asp?id=44&cat=Yamaha

Again will keep ya posted.

[BilgeBoy]

:) :) :) :) :) :) Smiles all round fella's

I raised the engine a hole and took her for a spin up the Logan today!!! Talk about different boat!!! Not only has the cavitation all but gone on turns...but the boat feels way more stable. I can now trim the bow in as well (without the foil)...oh...and there is no more spray going everywhere...just clean air.

Full throttle 5500 RPM about 35Mph...happy on 4000...She's Sweet man Sweet 8-)

Thanks to all those who voted "UP"...and not down (especially you Finga...you won the bet mate). I gotta tell ya I have had so much advice on this one... so called specialists who said..."If it's cavitating its gotta go down" and Boy racers who said "Ya need a 15 pitch Stainless". Oh well....pays to open ya ears and listen and then make informed choices.

Again...THANK YOU TO ALL who have contributed to this post! I owe ya one and hope that one day I can return the favour. Have a very Merry Christmas and a happy New Year...and safe boating over the holidays. Hope to see ya out there! :)

BilgeBoy

[Sea-Dog]

Hi Bilge Boy.

....................My experience with cavitation foils etc on many boats is that if motor height is satisfactory and foils are fitted
1 there is not too much water /spray being thrown around and
2 I have always been able to stick my fingers / hand between the top of the water surface and the foil when planing at speed

There is too much drag if the foil / motor is too low...................



Good luck, you'll get there in the end.

Cheers

Chimo

Just how many fingers do you have left after your experiments Chimo???? :o :o :o :o


- Sea-Dog

[finga64]

:) :) :) :) :) :) Smiles all round fella's

I raised the engine a hole and took her for a spin up the Logan today!!! Talk about different boat!!! Not only has the cavitation all but gone on turns...but the boat feels way more stable. #I can now trim the bow in as well (without the foil)...oh...and there is no more spray going everywhere...just clean air.

Full throttle 5500 RPM about 35Mph...happy on 4000...She's Sweet man Sweet 8-)

Thanks to all those who voted "UP"...and not down (especially you Finga...you won the bet mate). #I gotta tell ya I have had so much advice on this one... so called specialists who said..."If it's cavitating its gotta go down" and Boy racers who said "Ya need a 15 pitch Stainless". #Oh well....pays to open ya ears and listen and then make informed choices. #

Again...THANK YOU TO ALL who have contributed to this post! #I owe ya one and hope that one day I can return the favour. #Have a very Merry Christmas and a happy New Year...and safe boating over the holidays. #Hope to see ya out there! :)

BilgeBoy

Woohoo she's a goer (and no new prop) ;D ;D
Told you all the props OK
Another good result...beauty, now to the important things....rod in boat and off ;D

[Chimo]

Yo Ho Ho Sea Puppy!

When you write (print) your Xmas list, ask Santa for the big foils you know the ones that stick out a foot or more on each side of the fin, skeg, propeller thingo at the back of the boat. :)

Then you can play at the back of the boat too :D ;)

Merry Xmas to you too.

Cheers
Chimo

PS Aren't these smiley thingos great ;D

[Sea-Dog]

Yo Ho Ho Sea Puppy!

When you write (print) your Xmas list, ask Santa for the big foils you know the ones that stick out a foot or more on each side of the fin, skeg, propeller thingo at the back of the boat. :)

Then you can play at the back of the boat too :D ;)

Merry Xmas to you too.

Cheers
Chimo

PS Aren't these smiley thingos great ;D


Just tie my leash to the ski hooks, and off we go ;D

[blaze]

The idea of the foil is to skin on the top of the water (I dont like them), I would think to have any chance of that happening with what I am looking it needs to lift at least an inch, dont know if being so low its causing a tubulance around the prop.
cheers
blaze
ps have you tried with the foil off
my first answer to ya problem, glad it all worked out for you
cheers
blaze

[FNQCairns]

I just had a quick play with the prop calc and your numbers, you are doing ok, at around 10% slip with your prop at WOT, you could do better but it can get costly, my 30hp Johnson had a prop not much smaller than yours, interesting how Yamaha have a gear ratio on these engines that make the the prop spin around 25% faster at the same RPMs than my 30hp engine, hence the need for a small dia n pitch. I suspect your boat is about the biggest boat a person would want to fit the yam 50 to, by the numbers anyway.

Glad it's finally treating you well.

cheers fnq