Proper Motor Height

[Steve Taylor]

I have a 1989 BassTracjer 17 foot bassboat with a 115 HP Johnson outboard. I have been experiencing some porpoising and high RPM readings at relatively low speeds. The engine is mounted so that it is at the lowest point on the transom mount (i.e., the top bolt holes). Underneath the motor is an aluminum wedge plate that is 1.5 inches thick at the top of the transom and tapers down. If I try to trim up past 1/4 trim I start porpoising and get the high RPMs. I installed a hydrofoil fin on the cavitation plate and it has help some. I think I need to raise the motor two or three holes. Any thoughts...before I pay $$$ to have a pro look at it??<br /><br />Thanks,<br /><br />Steve Taylor

 

[JB]

Re: Proper Motor Height

Is your boat a Tournament V17 or Deep V 17? 115HP sounds like a lot of power (and weight) for a 17 foot aluminum boat. Check the CG spec. plate to be sure you haven't an overpowered boat.<br /><br />Your cavitation plate should be at the same height as, and parallel with, the bottom or keel with zero trim-out. Porpoising is a sign of too much trim-out, but if you are getting it with only 4 degrees out there is something wrong with the installation. I am very suspicious of that wedge. It sounds like a Rube Goldberg attempt to get around some fundamental problem. I could be wrong. <br /><br />Check out the above and good luck.

 

[Steve Taylor]

Re: Proper Motor Height

Thanks JB...<br /><br />I thought the same when I initally found the porpoising problem and saw the aluminum wedge. Mine is a Tournament 17 (fiberglass, wide transom, rated for max 120hp). The distance from the transom top to the keel line is 23 inches. When the motor is at zero trim the cavitation plate is about 21 inches from the top of the transom or 2 inches "above" the keel line. Now my boat has an inset transom about 17 inches below the transom top...it goes in about 6 inches and then drops 6 inches to the keel line. So I am not sure whether to use the 17 inch or 23 inch mark for placing the cavitation plate.<br /><br />Thoughts?<br /><br />Thanks<br /><br />Steve

 

[totallybored]

Re: Proper Motor Height

put your engine at mid trim and take a yardstick and put it along the bottom of the hull. The engine height is set when the cavitation plate is even or just below the top edge of the yardstick. As far as the porposing problem goes, you are overtrimmed up, you should trim down slightly until the porposing subsides. I would suspect that during tight turns that the prop also cavitates if you don't trim in all the way. Give it a try. Have fun.<br /><br />Allan

 

[JB]

Re: Proper Motor Height

I agree with Allan. Get the cavitation plate at the level of the keel and parallel with it at zero trim. The water stream coming UP from the keel is simulating trim-out and causing your porpoising. Good luck!<br /><br />JB

 

[Steve Taylor]

Re: Proper Motor Height

Hi J.B. <br /><br />If you look at my first followup I provide some measurements. My motor is lowered as much as it will go without adding a jackplate. At even trim I am 2 inches ABOVE the keel line and 3 inches below the point where the transom indents in. I may have to add a jackplate to drop the motor a couple of inches downward. Thoughts??<br /><br />BTW....thanks for the rapid responses.<br /><br />Steve

 

[JB]

Re: Proper Motor Height

Sorry, Steve, I missed the dimensions before. Whoever made that boat for BPS must have known that outboards with 23 inch shafts don't exist. That suggests to me that they intended the cavitation plate to be parallel with the transom step rather than the keel, but that doesn't match either 15 or 20 inch shafts. I still don't have a clue why they put that wedge plate between the motor and the transom. I doubt that the OEM intended the plate to be there, but if you take it out your problem will be worse. I have reached the limit of my understanding of the problem. Maybe OUTBOARDBOY has some ideas. Are you there OUTBOARDBOY? Help Steve out.

 

[Dhadley]

Re: Proper Motor Height

You never mentioned what prop you have. A 23 inch x dimention is ok with a step hull (does it have a pad bottom also?) but you need a prop designed for that height. IE -- a SST 2 will run a higher x dimention than an aluminum, a Raker higher yet and a four blade higher than a three blade. A good prop with "lift" built in (higher rake angle or extended cup) will help. Both OMC (Bombardier) and Stiletto make props that work well on your motor. They also may help your porposing to a small degree. Try rebalancing your boat by moving things around like batterys and gear. If you have two tanks you can play with weight there too. A larger hydrofoil will help. I've had a lot of luck with the King Fin. <br />Good luck!

 

[corm]

Re: Proper Motor Height

Transom wedges are common on stern heavy boats. Your motor hight seems ok to me.<br />I would check the hull for a hook or a rocker. Use a 4 or 5 ft straight edge and see if your hull is straight. Go from the transom forward to check to see if the hull is true. You may have a broken stringer in the hull also. If all is ok, like Dhadley says aprop may do the trick. 4 blade prop would be the best for holding the bow up on trim.<br /><br />------------------<br />...have fun...corm<br /><br />cubic inches = power<br />power = speed<br />speed = FUN<br /><br />Email me if you need to.