Jet boat tops out engine revs without planing

[ade843]

Can someone please help me, i am new to boating and i have recently purchased a Jet Boat with two U.A. Jet Drives made by U.A. Eng Ltd. Over the past ten years the previous owner has converted this boat from twin engines to a 1.5 Alfa flat four unit, the engine runs sweet and is currently running ratio 1:1 from belts direct from the drive. The engine will rev to approx 6000 fine but the propulsion dosnt really seem to increase, it just seems to bury the back end and produce alot of water disturbance from the back. I am starting to think that either the gearing is incorrect or the jet drives have either air ingress or worn parts. Can someone help?

Many thanks
Sam

[gonzo]

From what you are saying there are two jets run from one engine through belts. I would say it is underpowered

[daiquiri]

Hello,
Can you give some more info about the boat - like model, main dimensions, weight and deadrise?

[Ad Hoc]

ade

A jet will follow the prop cube law in terms of power, if your delivery is higher than it can absorb you have a problem; if your engine is revving higher than the jet can take, it will cavitate. You need to match the engine rpm with that of the jet.

Otherwise it is a bit like being on a bike, in say 1st gear when going down hill. You can't peddle as fast as the bikes wheels are rotating so you can't increase your speed. The bikes wheels being the jet and your feet peddling being the engine (going too fast but no delivery of power/thrust). When you select the right gear when going down hill, suddenly you get "power" back!

See the attached as a rough guide to explain further.

So you need geraing to match the jets power curve..

[Ad Hoc]

Just noticed your location..where on the IoW are you?

[ade843]

Im in Binstead, you?

[ade843]

I cant tell you much, it has Almarine on the wheel. I will attach some photos of the jets. Hope this helps

[ade843]

Hello daiquiri can you explain deadrise please? I will try and get you more information later today. many thanks

[Ad Hoc]

Im in Japan...but i know the IoW very well

[Ad Hoc]

Deadrise is the angle the bottom hull makes with a horizontal line that intersects it.

[ade843]

Still not too sure, i am very new to boating. Did you use to live here then?

[Ad Hoc]

ade
see this for an explanation
http://www.answers.com/topic/deadrise

yes, i grew up on the IoW, know it very very well

[ade843]

many thanks, where did you grow up then?

[ade843]

I have these pictures of the boat if it helps, its approx 16" long. Until i get home i cannot supply anymore info.

Cheers

[Ad Hoc]

ade

Nice looking boat.
I used to live in shanklin.

I think i recognise those houses in the background!!!