Porposing: Motor too high, or too low?

[Jer_H]

Hi all,

I have a brand new 17' Sylvan with a 115hp 4-stroke. I am presently running a 17" pitch prop which gets me up to 6200RPM WOT trimmed as much as I can before porposing. I can only trim up the motor a tiny bit, before porposing occurs. I feel that I'm plowing, and would like more lift, but no porpoising

From what I have read here, it seems that maybe my outboard is mounted too low on the transom. When trimmed all the way down, the aux. exhaust is submerged in the water.

The cav. plate appears to be just even with the bottom of the boat (deep v)\
I read that raising the motor could fix porpoising, however the mechanic at my marina says it will make it worse?

Any insight?

 

[joe009]

Re: Porposing: Motor too high, or too low?

you could try a set of hydro foils,a 4 stroke is a lot heavier.that sounds like your tbl.

 

[Woodnaut]

Re: Porposing: Motor too high, or too low?

a 4 stroke is a lot heavier

Joe might well be on to something here. If you could relocate some weight from the rear to the front, this would probably help the situation. Additional weight in the rear will make the problem worse. I have a 17' with a 90 HP 4-stroke and have to keep an eye on the trim. At 115 HP I bet that boat scoots.

 

[Bifflefan]

Re: Porposing: Motor too high, or too low?

With a 3 blade prop my boat (I/O) will get to proposing to an out of control state if I dont back off the throttle, at any trim. Using my trim tabs I can get it to act normal but requires a lot of adjustment.
I switched to a 4 blade and that has done the trick. Now dont have to use the tabs at all, but I do have them down just about 1/2" below flat and just leave them there all the time. This gives the best performance all around.

 

[gss036]

Re: Porposing: Motor too high, or too low?

A 4 blade stern lifting prop does as excellent job helping to stop proposing. I had to do that w/my boat when I put a new 225 Honda on. One thing to remember is that there are "Stern Lifting" and "bow lifting" props so just any old 4 blade prop won't do the trick. I have a 4 blade Merc RH that is a bow lifting type so is no use to me at this point because of the very reason.

 

[Jer_H]

Re: Porposing: Motor too high, or too low?

Thanks for the replies. I had the motor lifted one bolt hole and it's slightly improved, but I think I'm going to try a 4 blade stern lifting prop before anything else.

I still can't see my cav plate when near top speed and trimmed up, but I think it's b/c I can't trim up due to the porpoising. The cav plate is completely submerged.

We'll see.

Jeremy

 

[Jer_H]

Re: Porposing: Motor too high, or too low?

Well I just ordered a Solas Amita 4 blade in 17"pitch from iboats. I can't wait for it to arrive. I'm hoping that the superior stern lift it claims to provide solves my porpoising problem

 

[larlarsa]

Re: Porposing: Motor too high, or too low?

i also have this problem before,try to trim in as close as possible ,or add a transom wedge between transom and the motor,that's become much more better,at lost cost.

 

[Fisher3491]

Re: Porposing: Motor too high, or too low?

Hi all,

I have a brand new 17' Sylvan with a 115hp 4-stroke. I am presently running a 17" pitch prop which gets me up to 6200RPM WOT trimmed as much as I can before porposing. I can only trim up the motor a tiny bit, before porposing occurs. I feel that I'm plowing, and would like more lift, but no porpoising

From what I have read here, it seems that maybe my outboard is mounted too low on the transom. When trimmed all the way down, the aux. exhaust is submerged in the water.

The cav. plate appears to be just even with the bottom of the boat (deep v)\
I read that raising the motor could fix porpoising, however the mechanic at my marina says it will make it worse?

Any insight?

Yea, the mechanic at your marina is wrong.

We just got an new 18' Sylvan Deep Vee with a Yamaha F115. It was also mounted with the cav plate even with the keel. It porpoised terribly. We asked for it to be raised all the way up. They said to lower it down (only one more hole to go) to fix the problem. They tried it their way, no difference.

Got them to mount it as high as it would go, no more porpoising. Cav plate is over 2" above the keel and its still not high enough. Still to low in the water and too much drag. Looking into a jackplate.

Running a 19P prop and even with 3 people and full fuel its hitting the limiter before any trim. Need to try a 21P or maybe even a 23P

 

[cr2k]

Re: Porposing: Motor too high, or too low?

Your RPMs are too high. Add some pitch or ruin your engine.

 

[QC]

Re: Porposing: Motor too high, or too low?

Hey guys . .. one thing to be careful about in threads like this is assuming that any one fix is the right fix for all others. Many times porpoising is worse with a motor too high, many times moving it lower will help. Depending on the setup and the hull etc. apparently the opposite can be true.

Woodnaught is probably the closest to the main cause of porpoising though (I said main, not only) . . . weight balance fore/aft. I would search a lot more threads about porpoising, trim tabs, hydrofoils, trim, jack plates, chine walk, propellers etc. etc. etc. before I drew any conclusions in general on this topic. Way more complicated than "that's your problem"

 

[a70eliminator]

Re: Porposing: Motor too high, or too low?

Hey guys . .. one thing to be careful about in threads like this is assuming that any one fix is the right fix for all others. Many times porpoising is worse with a motor too high, many times moving it lower will help. Depending on the setup and the hull etc. apparently the opposite can be true.

Woodnaught is probably the closest to the main cause of porpoising though (I said main, not only) . . . weight balance fore/aft. I would search a lot more threads about porpoising, trim tabs, hydrofoils, trim, jack plates, chine walk, propellers etc. etc. etc. before I drew any conclusions in general on this topic. Way more complicated than "that's your problem"

I agree with the transom weight, I installed a 6hp kicker and it caused the boat to porpoise WOT, moved the manual trim one more hole in cured it.

 

[Jer_H]

Re: Porposing: Motor too high, or too low?

My RPMs are not too high. It's a 4-stroke outboard. It's rated 5800-6400RPMs as per manufacturer. I'm going to try the stern-lifting prop before anything else. Just waiting for it to be shipped.

 

[Jer_H]

Re: Porposing: Motor too high, or too low?

Well my 4-blade stern lifting Solas Amita prop arrived today, just in time as I'm heading up to the boat for the weekend. No extra brokerage fees either having it delivered to Canada! Can't wait to try it out. Hopefully this will stop the porpoising. I'll post my results.

Jeremy

 

[Subliminal]

Re: Porposing: Motor too high, or too low?

Good luck!

 

[oops!]

Re: Porposing: Motor too high, or too low?

how did you get around the fees?

i know iboats does a fantastic job for us canucks...but i have allways had to hire a broker

 

[Jer_H]

Re: Porposing: Motor too high, or too low?

I'm not sure if I "got around" any fees. The prop was $104 US. The S&H was $45 and with the exchange rate $160CDN was billed to my credit card. The order was placed on Sept 3rd and arrived on the 10th.

I was expecting to have to pay some added broekrage fees to UPS as I have in the past when ordering from US and UPS delivered, but this was not the case.

 

[Jer_H]

Re: Porposing: Motor too high, or too low?

Well, I was only able to get the boat out for about an hour this weekend with the new 4-blade Solas Amita prop. Although it was an improvement, the boat is still porpoising.

I agree that shifting weight to the front could help, but it's a brand new boat, and I barely have anything in it. I refuse to add weight to the bow as this seems like a band-aid solution.

I'm going to try raising the motor as far up as it can go and see if that helps. Failing that, trim tabs seem to be the most popular fix.

Jeremy

 

[ufm82]

Re: Porposing: Motor too high, or too low?

Here's a situation that may be occurring.
When an engine is mounted too low on the transom, the anti-ventilation plate runs submerged. If you look at the stern when the boat is moving, you see that the water that is coming from under the boat moves "up" when it comes out from under the boat. That's because the boat is in the water and the water wants to move back up to level. If the plate is underwater, it can cause your issue and here's why.
When the engine is trimmed down, the plate is angled so that it pushes the bow down. That forces water into the prop and this eliminates ventilation at take off. Once you are up and running you will trim up to gain speed. The plate moves and eventually runs level with the keel. You start to hear the rpm come up as drag drops and you move faster. You continue to trim up and then the porpoising starts. It does that because the plate is now angled up in the rear and it trying to lift the bow. As the weight of the boat overcomes the lift provided by the plate, it drops. Then the plate lifts it again and then the bow drops again. Waa-Laa- porpoising. This happens because the plate is completely underwater and is digging in as you move.
When you move the plate up to the proper level, it does not stay submerged. It instead rides on TOP of the water coming from under the boat. This causes pressure on the bottom of the plate which helps keep the bow from rising too much. Thus it does not rise and fall as before. This eliminates much of the porpoising you are experiencing. The plate should be 1" - 2" above the keel. Level with it is too low.
You said the 4 blade didn;t help. That's because it is not a prop problem; it is a set-up problem. The engine needs to go up. This will allow the plate to function as it is supposed to and also provides other benefits. It reduces drag which allows for higher speeds and easier planing. Without the drag in the stern, the hull will lift easier and give you better performance. Adding tabs to the boat will help to some extent as it will lift the stern even more but until you rectify the plate issue even they won't eliminate the problem.
Once you get the engine set-up right, all else you've done will definitely help. The 4 blade prop and tabs will give you even better performance than the stock set-up but again, do the required work first and then go from there.

UFM82

 

[Jer_H]

Re: Porposing: Motor too high, or too low?

Thanks for that explanation. I agree that the motor still has to come up more. I'm going to have it raised up a couple more holes and try that out. I'll post my results once its done.

Jeremy