Adjusting Trim

[potomac]

Okay powerboat gurus, when at full-throttle in my powerboat, having achieved a plane, with a fair bit of weight forward and nose not seeming especially high, not porpoising, there is a tremendous amount of spray coming either straight up or even back towards the boat on either side of the motor. The spray is quite high and I'm pretty sure this is not right. It seems a tremendous amount of wasted energy. Is this an issue of trim or maybe prop depth? I know a video or photo would be helpful. I'll try to get one this weekend if my verbal description of the issue is insufficient. Thanks- Aaron

oh: 25 hp Mariner motor, 14 ' V-entry flattening aft hull- (ahem) aluminum- (I'm working on it)

[wizbang 13]

The spray is coming from the transom or aft sides?
Is it a short shaft engine or is it maybe set too low? Anti ventilation plate at a bad angle? Is she steering ok? not pulling to one side? how fast is she going?
Putting weight forward is where the wasted energy is. HP is used lifting that weight. Better to trim a planing boat with the engine than ballast. Thus, power trim.
Alas, if no power trim, try raising the pin a notch.
14' tinnie with a 25, she might be close to "breaking out" which is where a boat stays at the TOP of the porpoising cycle, rather than being forced down.

[potomac]

Bruce- The spray is coming from the transom. 20'' shaft and the top of the prop is about 7 or 8'' below the deepest part of the transom. Maybe the prop is too deep? The transom is cutout- I assume for a 15'' shaft. Steering seems fine. The weight is forward because that's where the steering console/ seats are. It's human weight/ ballast- everything else is aft. Thanks- Aaron

[jonboy]

So many variables with fast planing hulls... I had a standard 40 hp ob on a fast launch with low transom.. very stable but would dig in a bit scarily on tight port turns . fitted a pair of those bolt on plastic wing things and the whole thing went to bits..., unstable unpredictablely at speed... also thought the back of the boat was a bit low in the water and a bit of a cock tail astern, so raised the motor by by building up the transom a couple of inches only... Different boat! but unstable at more than 30 kts in anything slightly choppy... so refitted the wing things and again total difference.. now stable and smooth, and predictable until there's a different payload. eg two 25 kg hounds sitting in the back who suddenly want to see something to one side..ducks or whatever...then it tends to dig in and pitch about abit. makes the mutts sit down tho'..!
I suspect small boats and various payloads and positioning have entirely different characteristics and you can't give a definitive answer. experiment alot,
Does sound like your prop's too far below the base hull line though,, looking at the drawings in the Yamahaha user's guide. Trim won't make a big difference but depth of prop seems to. Cavitation plate should be level with the hull or bottom of the transom for a start, and that's only three or four inches above the prop.
Even how much gas in the tank 50 litres is 50kgs more or less and where that gas tank is makes a big difference let alone a few six packs and a porky fishing pal. Ah..messin about on the river...!

[potomac]

A more careful measurement of the motor shows the anti-cavitation plate about 6'' below the bottom of the hull, so... I could try to build up the transom several inches or just build the Lumber Yard Skiff we have been discussing in another thread, or the Long Point Skiff I have lusted after for over a year now.

[Tom Lathrop]

Potomac is right. You have too much resistance down in the water and its causing the splash. Top of anti-ventilation plate should be about level with the bottom of the hull at transom. Deeper for slow boats and a bit higher for fast ones if you don't mind the risk of over revving the engine.

[mcdenny]

Sounds like you have a long shaft motor on a short shaft transom. Often small outboards have a five inch spacer bolted into the leg to convert a 15 to a 20" shaft. Any chance you can remove the spacer? There may be other components you'd need to convert, like a shorted drive shaft or water tube.

Denny Wolfe

[Breakaway]

Engine's to deep. What kind of boat? If modding the motor doesn't appeal, you could raise the mounting height simply by bolting lumber across the cutout. Also, what mounting hole ( on the engine bracket) are you using now. If you are using the top hole, use the bottom hole. You wont make up all the distance, but you'll make up a lot of it.

Kevin