Proping my boat

[FNQCairns]

Took the new boat for it's first test run yesterday and all the important bits worked nice the old 90hp yam is a sweet engine but doesn't much gut's but I expected that as a trade for fuel economy.

Actually one bit presented as a problem, in my rush I hooked the hyd steering back to front that morning:-[ but after 10min of re-learning was able to steer without thinking about it too much;D

Anyway the boat is great and is also dog on the water, it porosises around cruise speed and above, spray everywhere from the engine, trim has not much effect, so it felt like a barge, may as well just set it and forget:-/ .

None of it is stricly the boats fault, some people would fit a foil at this stage and put up with it but I will head down the setup path.


For those that are interested in this stuff, the on the water numbers were 4000rpm for 43kp/h and 5000rpm (wot:P, cut the test short because of this low wot) for 53kph GPS but some wind and a bit of flow, 1 person, light load.

The prop was a yamaha stainless 13 1/4 - 17p in new condition, 0 cavitiation on turns (didn't do a full ski type turn at speed for reasons stated above8-) ) so will be working to get some cavitiation as I set it up.

Ran the numbers and I have around 18% slip, so no hope of a boat that works well.

The cav plate is equal with the bottom of the hull proper (not extrusion) on this 6m pressed tinny and is on the second hole down, the half pod gives around a 2 foot setback to the leg.

Will be raising the engine to the bottom hole so 2 holes higher this week for the weekend and a run with the same prop.

Anyone want to guess what numbers I will get next trip with this Yam prop?
I also have a mercury 14dia-13p aluminium prop to I will throw on for a run anyone want to guestimate that one also (fair bit harder I know);D

leg ratio is 2.0

No prizes, just for fun:D

cheers fnq

[Spaniard_King]

FNQ,

my guess for outcome of 1 hole lift, engine rpm to be around 5350 and porposing to increase:P

I think your going to be dropping a hole and using a 15 as you have indicated a light load initially

cheers

Garry

[FNQCairns]

Hi Garry do you mean that later I will drop the engine down from the highest setting with the the 15p? if so I think you are right when the weight goes in things will change some, hoping the porposing will improve at least a bit with the extra height and disapear with the right prop -when I find it.
This 17p prop is useless to me now, been looking at the Hustler props from BLA any experience with these?

cheers fnq

[BM]

Got a picture of this boat? Particularly the rear end? Is the pod full depth? A stern heavy boat is the most common cause of porpoising.

[FNQCairns]

Hi BM no the pod is a setback type 1 inch high and full width.

My camera is not working properly but I have a couple of pics somewhere of the same hull that may show it, will post one.

cheers fnq

[FNQCairns]

Think this one is the 5.4m but it is the same design.

[BM]

Thatwould tend to rule out a bouyancy issue from the pod design then.

Whats the hull weight and the hull configuration?

[FNQCairns]

BM not sure what the hull weight is, from memory it was 350kg bare (may have been a bit more) all up now probably 750kg max.

Not sure what you mean by configuration it is a CC with an engine of 2/3 the max plate weight on it, relatively deep v for a pressed hull. Guaranteed if it was the cuddy, the up front weight would make the prop slip a constant and at least be delaying the problem till the higher speeds.

Suspect that slip is the issue I simply do not have the prop bite to overcome the bow dropping, hope Garry is wrong about ti geting worse when I lift and test but the proof will be in the pudding.

Do you want to have a guess how much extra RPM I will see? The test will be identical circumstance just 3L fuel lighter.

cheers fnq

[trueblue]

I reckon you will gain 300 rpm going up one hole on that boat and I wouldn't do anything more than one hole at a time

[BM]

Well your prop is obviously wrong given your WOT rpm. I would have thought 17 pitch on a 6mtr rig with a 90hp engine would be too tall....

I would switch the prop first and take some sandbags or concrete bags with you.

First run with the alternate prop fitted (I doubt this will do anything for your porpoising though).

Second run, take the weight bags with you. Store them amidships and add a bag at a time to work out the weight required to balance the boat. The bags stored amidships shouldn't affect the balance as you add/remove up front as required.

Or you could come back each time and grab another bag. Or go out with 2 boats and use the other boat as the mule to carry your weight bags.

Propwise you probably need a 13 1/4 x 13 to pick up around 800rpm. The 14 x 13 is probably going to be equivalent to a 13 1/4 x 15 at best. The extra diameter is as important as the pitch alteration.

Cheers

[FNQCairns]

Thanks trueblue for the punt, will see what happens, it will be nice to get this basic data on ausfish about porposing if it changes that and how many extra rpm it picks up.

Hi again BM yeah the prop is too tall I got it for a price that was a steal (ebay)and thought I could get it worked over in pitch, I rang the Cairns prop guy after I bought it to see if he would turn it into a 15p or 14p but he said the genuine Yam props are too brittle to re-pitch, so stuck with it now to get a base line (happy to hear if you know this is in fact true if anyone knows?).

So I then bought another bargain prop (ebay again) the alum 14-13 now with the plan to get it worked to a 13.25 - 14p is the next plan (guestimate). If this was an OMC V4 90 it would push the 17p but this little 3cyl, well is a little 3cyl:(.

Will be taking your advice and loading it up just not yet, need to recover from throwing fist's full of dollars at it before I purchase a quality permanent prop but plan to get it at least useable in the meantime within what is left of my boat budget - next to nothing:)

cheers fnq

[SO70]

sounds like the motor is way too low.
you stated there is a lot of spray from engine, which can be caused from a motor too deep.
the other thing is you have the cav plate in line with the bottom of the boat on a pod thats not in line with the bottom.
when you mount an engine on a non full depth pod the general rule of thumb is to mount the motor higher.
the water coming off the back of boat actually rises so there fore the engine needs to be higher.

[FNQCairns]

SO70 well read indeed, care to take a punt on what what rpm I will get on the second test raised to the bottom hole and whether it will calm the porposing?

Sounds like you think 2 holes will not be enough, I fabricated a slim bracket to mount this engine on so I had the option to go higher again - much higher, not this weekend though next weekend cause I will need to pull the engine to raise it above standard mount height.

cheers fnq

[FNQCairns]

repeat post

[FNQCairns]

repeat post

[Spaniard_King]

Just typed a huge reply and it went up in smoke....grrrrrrr

FNQ, IMO if you lift the engine the porosing will get worse as you are taking water pressure away from the cav plate. If you have water pressure on both sides you are likely to have less porposing as the water pressure is what holds the engine still whilst at speed. If you are going to lift it you will have less pressure on top as I doubt you are lacking pressure from below = more porposing. Chase the porposing then the prop wot rpm

just my opinion

Garry

[blaze]

excissive slip will cause porposing IMO, cant see how a cav plate will because it will only have atmospheric pressure on the top side when planeing because IMO it should be on the water line or slightly above
cheers
blaze
ps
maybe a cupped 15" prop

[FNQCairns]

Cheers Garry for your opinion I am of a different school where porposing is a function of prop slip, leg drag and a therefore the resulting lack of bow lift.

I consider this setup as it is now, not untypical of some dealer setups. Sort of equvalent to any engine fitted up to an inch below on lots of hulls.

Plan to see what numbers I can wring out of it everything identical, bit of a hard case this time around though with a low torque engine and a pressed wide bodied tinny, hoping to find an extra 500-600 extra RPM in the couple of weeks with the same weight, conditons and prop - be happy to just get 400 extra though.


cheers fnq

[FNQCairns]

yeah Blaze, if that is where I end up with a 15 and cupped, with a bit of luck may be able to jump to a good 16 from the US if the poprosing be-gone soon:)

cheers fnq

[Spaniard_King]

Blaze,

The water pressure I refer to is the pressure created on the cav plate whilst moving forward through the water

cheers

Garry

[SO70]

i would sort the spray problem and the engine revs out first.
have a look over the back of the boat when its going along to see where the cav plate is located.
if you see it buried in water its too low.
then after thats all sorted and it still porpising, put a fin on the cav plate.

[BM]

If the engine manufacturer required foils on the cav plate they would have manufactured the engine with a larger cav plate to begin with.

Foils are a bandaid solution to the real problem.

[chainsaw]

FNQ, Someone suggested weight up the front for test, I used 20 litre containers and filled them with sea water . You may have a weight distribution problem. I don't know but like all others will be very interested in the outcome.

Rod

[Pommy Matt]

fnq,
I had a similar problem myself recently with a yammie f60 on the back of a hinton 4.5 - porpoising when trimmed not too far out. Initially I thought it was just weight distribution (had cut front cuddy off & put 4stroke on the back) went the sand bag route - stopped the porpoising but loss of bow bouyancy not nice in following sea & loss of top end.
Did a lot of reading (not particularly experienced myself) & lifted motor from bottom hole (1 inch under) to top hole (about 1 1/2 inch above) - different boat. sandbags gone, porpoising gone, I don't have a tacho but a lot a lot more revs and top end speed. A/V plate is still under at plane (so is foil) - could probably go even higher!

matt

[FNQCairns]

Good to hear your experience mat, I have just finished lifting the engine to the highest setting and getting the hyd oil lines swapped around correct.

Hope to be where you are by the end of the w/end I went from equal with the bottom of the hull sheet on first test to 1.5inches above - highest stock setting, may need to go more before I am done while trying to find the base line.

Cheyne, will post when I have some numbers of value.

cheers fnq

[FNQCairns]

Went for test number 2, bottom of the tide with lot's of wind from squalls and then mostly 10 to 15kn gusts and all over the place.

numbers are

WOT 5600 65.4km/h =10% slip

5000 56km/h = 14% slip

4000 44km/h = 15% slip

At least this time due to less leg drag I was able to find the spot where this hull comes alive and lifts (above 60km/h) and with trim was able to hit the above wot RPM before I ran out of room, this was during a break between squalls and the wind lulled. No chance of ever getting there during test number 1.

The other two rpm numbers are a bit rough, due to the wind when compared to test 1 and MAY be a fraction better in reality - or not! it was a mess out there today.

Porposing has calmed down to a level I can now power and trim through it unless heading directly into 20kn wind at cruise, couldn't do this in test number 1 even with near no wind.

Still couldn't force any ventilation (cavitiation) no matter how I tried so I am still too low even an inch or more ! Next weekend will hire an engine lift and raise the whole kit and caboodle with the jack bracket I made and test again hopefully on sunday.

So the upshot is proposing 60% better, not gone but controlable, trim 80% better no longer a barge, speed better also:) all round I had fun this time, not bad for 15min work raising the engine.

Stay tuned for test no3, next sunday same prop, same stuff and fuel in boat only difference will be another inch higher in engine height - an inch higher than the highest stock setting.

cheers fnq

[Chimo]

Hi FNQ

Sounds good, I guess you got the PM.

Chimo

[FNQCairns]

Thanks Chimo got the PM, will see what the start of the week holds.

Very happy, it behaved at least like a boat this trip:)

cheers fnq