porpoising and reverse rocker

[boaring]

hey fellas new to the forum and after some help from those far more experienced than myself. I recently purchased a used seascape kingfisher 482 with a 75 honda in absolutely immaculate condition only 42 hrs on the motor, this rig porpoises badly, fitted a foil and cav plate is level with hull bottom, i can reduce porpoising to an acceptable level by moving weight forward and trimming in but at the detriment of fuel usage and optimum performance etc. There isa 20mm deviation(rocker) in the back half of the hull. I recently contacted the manufacturer and was informed that they build 25mm of rocker into these and other hulls in their lineup, and basically told me my boats issue was stern weight ie my 75 honda about 165 kg which my boat is rated for is causing the problem, and a set of trim tabs will solve the problem this all sounds a bit suss to me, so im pretty keen to speak to anyone with a similar setup and get their opinions as i know a lot of these hulls are sold with this motor or 80 yammies which are slightly heavier any help would be much appreciated cheers.

[STUIE63]

firstly take the wing off . then does the honda reach max revs and when on the plane is the cav plate just skimming across the top of the water if your cav plate is level with the bottom of the hull it sounds like your motor is too low
Stuie

[Crocodile]

Hello boaring,

At what speed does the porpoising start?
Does the boat drive properly at lower speeds?
Is it possible that the 75 simply pushes the boat too fast for its design?

[robothefisho]

If it was me,

1) start lifting the motor until the cav plate is clearly visible when planing, with a little bit of water splash. The higher you can get the motor the less bow lifting effect it has.

2) Four blade prop.

3) Try taking the foil off, as it really shouldnt make any difference once the boat is planing as it should be clear of the water surface.

[Chimo]

Hi B

Had a friend with a short boat with a heavy motor on it and it did similar things. Not a pleasant way to go boating. The boat used to have a smaller 2 stroke motor previously and was better but still not ideal. He sold it.

You could try and fiddle with motor height; you need to post pics of what the present set up looks like before people comment tho.

Cheers
Chimo

[brrbear]

If the cav plate is level with the hull as you say, this is probably the issue.

it is the first small plate above the prop that should be level with the keel.
Was the boat set up for twin motors with short shafts then fitted with a long shaft ? which has left you in nowhere land.. I have seen this before.-----------fairly simple to fix

[BM]

Engine too heavy.

[FNQCairns]

So it has a hook? At the engine porpoising is the prop grabing then slipping, slipping each time the nose of the boat cannot beat gravity any further and it just falls. If your prop had the grip at this stage then the nose would stay up there happy as.

So first thing is to ensure you are achieving maximum thrust at the speed the nose falls, so prop it well, if your proping is a little sus even a little the porpoising will be harder to beat, prop it higher even than the specs you don't need to actually go there.

If you are proped well then fit wedges, this is an almost unbeatable combination when there is no further good choice....like you cannot go any higher with the engine sensibly or as you state you fitted a foil.

lastly a 4 blade prop, the extra surface can give muscle where you might need it most to overcome gravity/slip.

[Jarrah Jack]

My mate has the same boat with an 80 4 stroke yami and hasn't had any problems with it. He drives it like a cut cat so would've been quick to find that type of issue. If it does happen to be the weight is there anything like fuel and batteries you can move forward to counter the problem.

[boaring]

thanks for the replies and help fellas im realy struggling to get this sorted.
STUIE i have taken the foil off and will give it a run today without it, i get about 57 5800 rpm 2 up trimmed out but cant maintain it because the porpoising gets real dirty to the point of being dangerous, can run trimmed out at lower speeds but still bad porpoise going on so ultimately i trim in to reduce this, yes have thought to raise motor 1 hole up but was advised by seascape that height should be fine where it is and he actually guessed my motor size and height before i told him so there must be a few setups like this around.
CROCODILE yes still porpoises at slow planning speed but handles ok when not planning and im quite happy to cruise around in the midrange but trimmed in to slow the dolphin reduces fuel effiency quite a bit.

ROBTHEFISHO 4 blade prop how could this help, stern lift maybe.
CHIMO will have acrack at posting pics.
BM theres a lot out there same as mine, a 60 4 stroke would be marginal on this hull and save about 45 kgs not realy that much weight difference cant figure out why mines so bad, manufacturers telling me to fit tabs so hes basically saying that all these hulls with similar motor setup will porpoise and you need to promote stern lift to compensate for weight im concerned hes telling me lies to cover for a hull defect.
FNQ no hook but rocker the hull rises from last third to transom by about 20mm i have 2 props a 15 and 17 only used the 15 so far but 4 blade sounds interesting
wedges im not sure how they work what could they achieve?
im not real keen on drillin holes for tabs especially if it dosent fix my problem
i would love to hear from some guys with this boat and setup as this is a pretty standard rig sold by seascape dealers thanks again for the help cheers.

[boaring]

jarrahjack the plot thickens an 80 yammy is even heavier than my honda by about 10 kg i think this guy could be tellin me lies but yeah i do move weight forward and this does help but just bandaids the problem cheers.

[Fed]

http://www.seascapeboats.com.au/page/news_&_info/1_R.I.S_%28Rocker_Induced_System%29.html
I'd take a hell of a lot of convincing that this is a good thing.
Here's our boat but you have to fit trim tabs my arse.

[Lachie1]

I think you are going to have a battle on your hands, a mate of mine is still going through this same problem with another manufacturer of a similar sized boat. (and has been for over 2 years) I find it very hard to believe they build a boat with 25mm of rocker... I think they have lied to you about this to keep you off their backs - my opinion only. Ring a marine engineer and talk to them as there is alot more to it than meets the eye...

Trim tabs will fix the problem - the problem being porpising and the cause being too much rocker. There is still a problem with the hull however. A boat that size with a motor within the correct weight range and set at the right height does not need trim tabs.


Regards,

Lachie

[Angla]

I have a set of smart tabs that may help in your circumstances. Look up SmartTabs on line.

Pm me if you would consider them. I would probably want $50.00 for them.

Cheers
Chris

[OPTI]

trim tabs is proably the easy fix ,but since you dont want to go down that path ,you need to shift weight ,now contrary to popular belief ,shifting the weight aft is more succesful than shifting foward ,think about what the boat is trying to do ?its trying to lift the bow but gets to a certain point and carnt maintain the lift and it drops,creating the porposing ,putting the weight forward will never let the boat sit back on the rocker and will end up in poor performance and a usually wet boat.
id buy a couple of bags of pool salt 20kg each and start with them at the transom and go from there,once you get the desired weight distrubution then start moving things around to get the desired balance.

[Shawn 66]

Not on topic

[johncar]

hey fellas new to the forum and after some help from those far more experienced than myself. I recently purchased a used seascape kingfisher 482 with a 75 honda in absolutely immaculate condition only 42 hrs on the motor, this rig porpoises badly, fitted a foil and cav plate is level with hull bottom, i can reduce porpoising to an acceptable level by moving weight forward and trimming in but at the detriment of fuel usage and optimum performance etc. There isa 20mm deviation(rocker) in the back half of the hull. I recently contacted the manufacturer and was informed that they build 25mm of rocker into these and other hulls in their lineup, and basically told me my boats issue was stern weight ie my 75 honda about 165 kg which my boat is rated for is causing the problem, and a set of trim tabs will solve the problem this all sounds a bit suss to me, so im pretty keen to speak to anyone with a similar setup and get their opinions as i know a lot of these hulls are sold with this motor or 80 yammies which are slightly heavier any help would be much appreciated cheers.

Have come across this alot over the years not so with my own boats fortunately but with friends, and found that it is often a very fine line between porpoising or not and it should be easily fixable hopefully.
the most common fix that has been successful is to raise or lower the engine mounting height but without knowing where the engine is right now and from what you said I would try raising the engine as some others have also suggested.
By you engine revs it seems that your prop is pitched about right, but changing to a 4 blade could help but do all the cheaper options first and talk to the guys at Soloas as they are knowlegible and may be able to help with suggestions as well.

I know that you have been told it's the right height by SeaScape but I have heard it all before and proved it to be wrong. There are more variables that they account for at times. Even with my new boat I raised the engine against the dealers recommendation, not for porpoising but for slight lean to port underway. My boat is much happier at the new height, plus I changed the prop to what they originally fitted. Solas Props were very helpful with that too I should say.

Interestingly a few years back a mate of mine and I both had the same Haines Hunters 680's. same engine too. Mine ran dead straight all day long yet his porpoised. The only difference I recall was that mine had a stingray foil and I think his was one hole lower on the motor mount. he raised the engine and fitted the same foil and it was cured, but I think it was due to the height more than the foil but the foil helped to give lift out of the hole.

Another friend bought a new Seafarer Viking and yeah porpoising all day, same thing had an arguement with the dealer who was determined that the engine height was correct, but regardless against the dealers advice my friend raised the engine one hole and problem solved.

So that is a reasonably easy and reversable no cost thing to try first if you feel confident to do it yourself.
It should at least show an improvement and make some change hopefully not worse. It should give you some clues at least as to what may be going on.
The cav plate should be skimming the surface if the engine is running at the right height on most planing hulls, although they may need to run a bit deeper on some flatter hulled tinnies with pressed bottoms due to cavitation.

On the average boat, the water rises off the bottom at cruising speed at roughly 1" higher for every 12" behind the back edge of the planing surface so factor that in as well when estimating a starting point for the cav plate.

It would stand to reason also that if you are approaching your transom weight limits the boat may benefit from the engine being mounted a bit higher. Can't hurt to try and be very careful if you are checking cav plate location or prop performance while underway. Better to have a friend at the helm.
If I do any checks on mine while on my own I tie a rope to the kill switch lanyard so if I slipped or fell at least the motor would be shut down.

Anyway all boats are a bit different and not one cure for all but I am sure you will get it sorted with a bit of trial and error and asking plenty of questions.

[robothefisho]

Boaring,

4 blade props, in general, bit of stern lift, more thrust, quicker to plane, hold a lower planing speed more grip.

You may find you don't need to go as far as a 4 blade prop, just lift the motor untill the problem is solved or ventilation becomes a problem. If ventilation becomes an issue before the propoising is solved try a 4 blader. Also make sure your getting to the high end of your engines rev range.

[Moonlighter]

I agree with the suggestion to lift the motor one hole higher to start with. It may even need to go up 2 holes, but only an on-water test will determine that.

My previous boat was a similar size to yours and even though it had a yammy 70 (so was a lighter engine than your Honda) it did porpose especially at mid-higher speeds. Lifting the motor one hole made a definite improvement.

By the way, you cant really be sure about correct engine height by just looking at the cav plate relative to the hull when the boat is on the trailer. The only certain way to be sure is to do what the guys have stated earlier - take it out, find a smooth straight area (like a river) run it at a good cruising speed at mid trim settings, and then with a mate at the helm carefully go astern and have a look - the cav plate should be just skimming the surface, maybe a bit of water splashing over it - but if there is a solid flow of water over the top of it, it is definitely too low and should be lifted.

A trick to help you see the plate better when underway like this is to put a red stick-on dot on the trailing edge of the cav plate.

I lifted my motor simply by winding the jockey wheel up on the trailer with the motor trimmed down until the skeg was sitting on a piece of 50x75 pine on the ground, then undid the engine bolts, tilted the engine back so it came off the bolts (helps to have a mate help you keep it all steady.

Then had another deckie wind the jockey wheel until we were lined up with the next higher engine hole, then slid/tilted the motor back onto the bolts and put the nuts back on. Pretty easy and safe way to do it if you havent got a engine lifter. Takes half an hour.

Cheers

ML

[albey]

Ive never heard so much bullshit in all of my life.Only an idiot would design a boat with inbuilt hook.If you have ANY hook the boat WILL porpoise.Aluminium boats set up on the trailer such that the transom overhangs the rear roller stand a chance of incurring hook and even 5mm will cause you problems and some of the roundabout solutions will go a long way to curing this amount of hook,BUT 20-25mm is an insurmountable problem and will not be cured by these wayout money saving means,the ONLY way to fix the problem is to fix the hook.If it was me I would go to a marine surveyor for his written assessment(shouldnt cost a lot)(we use them all the time to get ammunition for warranty work to be carried out if the dealer wont accept the advice of our shipwrights)then with that go back to your dealer who have sold you a dud for a fix.This fix will be expensive.Sorry

[OPTI]

albey the op said the hull had a rocker [opposite to hook]

[albey]

albey the op said the hull had a rocker [opposite to hook]
Bit ambiguous really.The headline says REVERSE ROCKER which is WEDGE or HOOK.Its all irrelevant because they both cause porpoising and the solution is the same>FIX THE BOTTOM
An interesting article

http://books.google.com.au/books?id=VQEAAAAAMBAJ&pg=PA80&lpg=PA80&dq=boats--reverse+rocker&source=bl&ots=ocKi05zzfy&sig=_WdWmvUMxapYGGngXDxR279FY3w&hl=en&ei=jpWqTeeNM4eWvAOdoMT_CQ&sa=X&oi=book_result&ct=result&resnum=7&ved=0CEYQ6AEwBg#v=onepage&q&f=false

[OPTI]

not really albey ,some boats have rocker designed in them like the 233,it is a black art in hull design,rocker enables the hull to rock foward at slower speeds and give a softer ride in the rough as well as being able to rock aft at higher speeds giving the hull better speed and economy.
hook causes the bow to pull down,i know of some hull designs that have hook built into the keel for that very purpose.
i wouldnt really say either are bad ,both have fores and againsts,its just a matter for getting the balance right;)

[Fido]

To the original poster of this subject. I had a boat built by Belpray Pty Ltd/ Spacecraft Boats at Toronto NSW There were many issues with the boat and the boat builder treated me with contempt. The boat was ordered with a 20 degree deadrise and when the boat was delivered it had a 16.5 deadrise. The windscreen didn't meet correctly, the paint job was terrible. The bottom had a mixture of 5 and 4 mill sheets whereby the contract stated 5 mill. Some of cosmetics of the welds were unsatisfactory and the trailer that was made by them was also unsatisfactory.

All this aside the boat had a rocker in the bottom of the boat. The builder treated me as an idiot. It was only after we instigated decisive legal action supported by two indepenmdant architecural firms that the builder capitulated at the mediation table when he was informed he would loose the court action.

The boat went back to the builder and a refund was given.

The main result of the rocker is that it causes a boat to porpoise at a certain point. ie It planes on the bottom, then on the rocker bit and then because it runs out of planing surface because the rocker is a deformity created in the building process. In my case rather carelessly. A boat shouldn't have rocker. You can search this subject on google to the enth degree and on very rare occasions a rocker may be built into a high performance craft and also into some canoes.

The motto of the story is:

Because of what happenned to me I wouldn't recommend to anyone to buy a Spacecraft Boat. The male owner was less than truthful to me.

Secondly get a long straight edge as I believe the last one third of your boats planing surface should be straight.

Fido

[boaring]

gday fellas thanks again for the replies, i raised the motor 1 hole today but didnt get out to water test it, hopefully tommorow, its funny because everthing i have read on rocker in hulls via the net has basically stated that its a defect and causes these obvious problems but as you can all see FED posted up the seascape web address which states they factor rocker into the building process of their hulls to increase performance, but i cant understand why other builders dont do this, makes me think that every kingfisher with a similar weight 4stroke on the back should behave like mine( very strange) and mines well out of warranty but that wont stop me huntin em down if it is a defect causing the problems, would sika flex keep smart tabs in place just for a trial run it may prevent me having to drilling holes in my transom if they dont work.
Cheers.

[STUIE63]

get the motor height and prop right first , so they are eliminated from the equation then start thinking about other things

[Chimo]

Hi Boaring

Having a rocker should not be a problem, in fact a plus. Most builders don't do it as its harder but done right its great. I had a 5.6 al with one and it was great, never porpoised and ran well. Ditto with a couple of 5 m units all fitted with max HP

Do one thing at a time as suggested above, set the motor height; go up till it cavitates / ventilates then down one hole. I set the 5.6 up in fresh and at sea if driven too hard into corners the prop would lose it. The difference was the bouyancy variation between salt and freshwater.

Then think about a foil on the motor, dont worry about drilling a couple of holes.

if your going to fit trim tabs which are worth having based on my experience, on anything over about 5.5m long, fit real ones that you control like Lencos or Bennetts. If the boat need to be longer Lencos or Bennetts in the up position will give that to you as well. Or you could just add some pods like I did to make my 10 footer a 12 footer; runs good and can carry 100kgs out the back no sweat.

Cheers
Chimo

[boaring]

hey chimo everthing ive read states builders dont utilize rocker because it causes handling problems like mine, in fact rocker from what i can gather is seen pretty universally as a hull defect and is rarely seen exept in some very high performance speedboats and not recreational fishing boats, funny i asked a local boat shop owner about trim tabs the other day he then asked how big my boat was i told him 4.8 he then laughed and said youve got a tabs havent ya i said ive got a seascape which is made by tabs, looks like they have a reputation in regards to porpoising in fact the more i look into this the more i realise i may not be able to fix this boat with setup changes after all. cheers

[Chimo]

Refer to post No 5

Hi B

Had a friend with a short boat with a heavy motor on it and it did similar things. Not a pleasant way to go boating. The boat used to have a smaller 2 stroke motor previously and was better but still not ideal. He sold it.

You could try and fiddle with motor height; you need to post pics of what the present set up looks like before people comment tho.

Cheers
Chimo

[Spaniard_King]

I have seen this Issue fixed by welding alluminium wedges on the underside of the hull, its the easiest, cheapest fix for this problem.

[Salad Fingers]

So does this boat have a rocker or a reverse rocker? Im confused.
My 12ft tinny has a rocker, not by design - just by being very roughly treated. Doesn't porpoise though with the motor mounted up nice and high

[boaring]

it has 20 mm of rocker i took it for a blast today after moving motor up 1 hole and removing the foil and it made a big difference, ive never been able to trim out as far as i did today without turning into a dolphin i will try it with some more weight over the next couple of days but the difference is remarkable given the motor was raised only about 20 mm, i was stocked i tell ya cheers for all the help fellas

[Rodpal]

hey boaring

i'm no expert but from what i understand a negative rocker in the bigger platy/ glassies - inboards come into play when the big girls go into a turn otherwise they would want to just lean over and go steight ahead

my 420 has a flat planing area through the hull and you can trim the motor either way of ok and feel the difference

my 6.5 / 200 suzuki has the same and you can feel the boat trim through your feet and a very slight pitch in the motor but again you can trim the motor either side of somewhere in the middle and feel the diference

both have foils and the best additions to the boats i've ever done .. other than my beautifull wife .. and yah man shes right here .. ok coast is clear hehehe .. now what was the question haha ok all good, yah like i said i'm no expert but cant say i've seen a small boat with a rocker

somehow hope this helps and welcome to ausfish man .. second best thing i've done .. and yah the coast is clear lol

cheers rod

[STUIE63]

it has 20 mm of rocker i took it for a blast today after moving motor up 1 hole and removing the foil and it made a big difference, ive never been able to trim out as far as i did today without turning into a dolphin i will try it with some more weight over the next couple of days but the difference is remarkable given the motor was raised only about 20 mm, i was stocked i tell ya cheers for all the help fellas
when you are on the plane is the cav plate just skimming the top of the water or is it still under ? if under the try another hole up . in my experience alot of boat shops put motors on too low so that the owners don't come back whinging about the prop ventilating/ cavitating on hard turns . it is good to hear that the boat is starting to behave better . but keep going with getting it right and you might be surprised as to how good it can be
Stuie

[Chimo]

What Stuie63 said and as said in post 27

"Do one thing at a time as suggested above, set the motor height; go up till it cavitates / ventilates then down one hole. I set the 5.6 up in fresh and at sea if driven too hard into corners the prop would lose it. The difference was the bouyancy variation between salt and freshwater."

Cheers
Chimo

[johncar]

That is great news that you are getting better results.
I think that is a relevant point Stui63 made re the "dealers don't want customers coming back whinging re the cavitation" in harder turns.

For my way of thinking and I have been setting up boats for a long time. I know that when the engine height is at it's optimum you can expect some cavitation in a medium to tight turn with your motor trimmed out in cruise trim.

This is not a problem and unless it's excessive that is where I would want it. So if you intend on doing a tight turn just trim the leg in as you go into the turn which should minimise or stop any cavitation.
If you get cavitation with full trim in well then the engine would be sitting too high.

So it's just finding that happy balance and your engine doesn't have to be able to use the full trim range, like with my boat I run the engine as high as possible and say above 3/4 trim out I will start to form a rooster tail as the prop pushes the water to break the surface but that is fine because I still get ample bow lift in any conditions and have all the benefits of running the prop as shallow as practical.

[robothefisho]

it has 20 mm of rocker i took it for a blast today after moving motor up 1 hole and removing the foil and it made a big difference, ive never been able to trim out as far as i did today without turning into a dolphin i will try it with some more weight over the next couple of days but the difference is remarkable given the motor was raised only about 20 mm, i was stocked i tell ya cheers for all the help fellas


Thats great news Boaring, it's always good when the simple things solve the problem.