Engine Height Question

[metriccrescentwrench]

I've got a 15 ft fish-n-ski with '81 Johnson 90 HP. At full throttle I'm seeing about 5600 RPM and about 42 mph on my gps. I was always pretty happy with that, but after talking to an acquaintance I'm wondering if that can be improved upon.

He's got the same hull as me, only his is an '86 with a 90 HP Yamaha. He gets about 51 mph (gps) at 6000 rpm, he told me he saw a lot of improvement after raising his engine.

Since my engine is currently on the lowest bolt hole it's had me thinking about tinkering with the engine height. But before I do that I'm wondering if there are any characteristics to look for to suggest that performance would benefit from the engine being set higher on the transom.

In the case of my boat, when I trim the engine up (at full throttle), the boat never shows any tendency to porpoise, I just hear the prop start to ventilate at some point slightly past the trim angle for maximum speed. I'm no expert at boat setup, does this have any significance?

BTW, my engine has fins on it. Previous owner put them on, I like how quick the boat jumps out of the water (great for pulling skiers) so I've never bothered seeing what happens without them.

Thanks in advance for any insight. In my case it's not a trivial matter to make an adjustment and then haul the boat some distance to the lake, hopefully to find that the boat still handles properly, so I'm trying to get as educated as possible ahead of time.

 

[Dhadley]

Re: Engine Height Question

The correct engine height (X dimension) for any boat / motor combo depends largely on what prop you run and how that style prop works with the hull design. It's very possible you both could be running the same pitch but the prop styles are very different.

All things being equal and providing your prop style and condition like height, it's way possible to gain by going up.

What brand & style prop do you have now?

 

[tashasdaddy]

Re: Engine Height Question

the av plate should be 1-2 inches above the keel. just watch that you still have good water pickup.

 

[metriccrescentwrench]

Re: Engine Height Question

What brand & style prop do you have now?

It's a 13-1/4 x 17 inch aluminum prop, I believe it's the original prop that came with this boat/engine but don't have any way of knowing that.

 

[Dhadley]

Re: Engine Height Question

Typically what we see is that an aluminum prop won't run as high as a ss one. Then you get into types of ss props. There are those that like more height than others and those that have more lift than others. Some have bow lift, some have stern lift.

You're at less than 10% slip (if the GPS is accurate) which is pretty darn good. What we'd be looking at if we were looking to improve your set up is going with a ss 19 or possibly a 20 that has a lot of lift. Lift will reduce the wetted surface which reduces drag which means an increase in efficiency.

Make one change at a time. Raise the motor with the prop you have and see how the hull responds. You have a good baseline, write down everything as you test after each change.

 

[jevery]

Re: Engine Height Question

That's why the bass boat boys with jack plates usually have a water pressure gauge. Not hard to burn one up at the upper limits. One indication that you've gone too far is that you'll vent before achieving sufficient bow lift. It's not hard to over do it. I'd go a hole at a time if you have the means to do it at home. You might measure his (Center of bullet below pad) and compare to yours before making changes.