Porpoising and engine height

[tony_cliffy]

Re: Porpoising and engine height

Forktail,<br /><br />I haven't raised the engine yet - still working on the logistics/tools/buddypower aspects, but I will soon.<br /><br />The boat is basically a very large johnboat but it does have a semi-V bow and a little bit of a keel. It is 20.5 long and has a 96" beam. It would be difficult to relocate the existing items (fuel/batteries) but could I simply add some weight up front? Or would adding weight for the sake of weight not be advised? I'm thinking of filling a forward bait tank with water or maybe stowing an extra concrete anchor. <br /> <br />

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[Forktail]

Re: Porpoising and engine height

Very nice tonycliffy.<br /><br />The mounting does appear to be way too low...by about 3-4 inches.<br /><br />I would guess you're going to lose a lot of drag and pick up plenty of rpm and speed when you get the mounting height right.<br /><br />If it still porpoises after that, you can try adding a little weight to the bow to help lift the stearn. Sometimes it only takes 10-20 lbs, or an anchor and some gear. Next would be propping.<br /><br />Good luck, and let us know how it turned out.

 

[LubeDude]

Re: Porpoising and engine height

Man, your engine is way too low, you may even have to have some kind of a transom spacer made, Ive had to do that before. Also, Im not sure I would want to go a lot faster in that boat. After you raise the engine I think you are going to get real close to 50MPH out of it with the right prop. At that speed in a light boat like yours, you could catch air very easily with a basically flat front. I think that raising the engine is going to help with the porposing problem, its going to be interesting to follow your progress. We are all going to learn from your experience!

 

[tony_cliffy]

Re: Porpoising and engine height

Lube,<br /><br />That picture is a bit deceptive and makes it look worse than it really is. I don't know - I think it has to do with the angle I took the photo. Regardless - it should come up.<br /><br />I have a new concern (as if I don't have enough). JD touched on it with his instructions. My transom has a cap that goes along the top edge but the motor has a flat portion in the back that will run into this as I slide it up. I am not sure whether to just pinch it or what. I can tell I need to go up 2 bolt holes, but doing so will have that flat piece of the engine against the transom up on the bottom edge of that trnsom cap.<br /><br />

 

[Forktail]

Re: Porpoising and engine height

I'd leave the transom cap the way it is. It doesn't look that thick, and the engine bracket should be ok on top of it. You could always grind a little off the engine bracket, but I'd leave the transom alone.

 

[LubeDude]

Re: Porpoising and engine height

Ya, if anything it will tuck your engine under some and might be benificial not allowing you to tim quite as much. Minumal though!

 

[imported_JD__]

Re: Porpoising and engine height

tonycliffy,<br />The reason I knew about the transom cap is because I own two aluminum boats that have a similar type cap. Remove the top bolts then loosen the bottom ones enough that you can slide a wooden paint stirring stick or thin shim over both lower bolts between the bracket and the transom. gently "trim up" to get the top back enough to clear the cap. In other words your engine is loose enough that when you gently tap the trim switch the relationship between the bracket and the transom changes, not the angle of the engine. This will keep it from hanging as you jack the engine and the shims will protect your transom. Once you get it to the right height, remove the shims, install and tighten the bolts. I looked at my transom cap after raising and lowering my engine several times, there was hardly a mark on it. I have a Johnson 4-stroke on my boat and really appreciate the slotted lower holes. Once you do it a few times its a piece of cake. With your dearly beloved's assistance it only takes about five minutes.<br />JD

 

[Jdeagro]

Re: Porpoising and engine height

As everyone is saying you will need to move the engine up and this should help. My guess is that it will still porpoise. Let me know if it does.

 

[tony_cliffy]

Re: Porpoising and engine height

It has been a couple years but I wanted to follow up and say thanks for the valuable input. I *did* end up raising my engine 2 bolt holes very easily using the technique described. My boat performance improved in both speed as well as a little help with the porposing. A lot of the porposing had to do with learning where she wanted to be trimmed. I think I was overtrimming to achieve max speed, but eventually I learned what she could do and usually run below WOT and don't get the porposing.

I have used this info to help a couple other folks with their engine height mounting issues which is why iw as re-reading this today. Thanks again!

 

[LubeDude]

Re: Porpoising and engine height

Thanks for filling us in, so many times people post and then leave us in the dark. Are you still thinking about the trim tabs? I have them in my Bass boat and would never even think asbout taking them off. They are great for pulling a tube as you can get on plain very quickly and stay on plain at speeds from 10-15 MPH which is really nice with small kids.

 

[Bwalker]

Re: Porpoising and engine height

Jon style boats are notorious for popoising. Mine did it till I found out how to fix it without resorting to tabs and other devices that can decrease speed.
What you need to do is to increase stern lift, by "hooking" the hull. A racer of some merit that holds national records turned me onto this. he hangs out at www.refugeforums.com and uses the ID 1995US1. He can help you more.
In short it involves forming the back of the hull with a piece of wood and a sledge hammer. Sounds crude, but it completley transformed my boat.