Jet/Engine/Deep V hulls

[Sea Zar]

Hello Everyone….
I’ve been enjoying the posts in regards to jet units & their inherrent issues. Though not fully specific to my very own devastating problem. Devistating, from the point of….me buying a “lemon” of a boat & having to spend 6 years re-building it & re-designing it to come to a possible conclusion that a the Jet unit might not suit my boat. I have spent thousands of dollars rebuilding a jet & 4 engines in one year due to detonation problems, big end bearing problems & plaining issues. This is the first boat I have owned & had no experience in boat building. Now give me anything & I’ll make it out of fiberglass.

Before I get started I’ll give you a basic run-down of the specs.

21 foot fiberglass half cabin cruiser called a “Savage Marlin” Designed for offshore fishing in Australia. Some say one of the best designed boats ever built alongside the Bertram. It’s the only reason why I didn’t dump it at the rubbish tip.

Deadrise is 22 degrees & is a plaining hull
Weight with engine & fully tanked is approximately 1.8 tonne

Engine is a 350 mercruiser Chev with approximately 9.5:1 comp ration, mildly warm cam (stage 2) roller rockers, Marinised Holley 600 vac 2ndary etc.

Used to have a Volvo Penta I/O

Jet unit is a Doen Jet (Australian design) fully external & is a direct 1:1 drive.
Impellor diameter is 8 inches. Pitched up (don’t know how much but I do know that it is.)

My situation is this.
Even though I have spent big bucks on a new & improved engine & creating maximum torque at 3000 to 5000 rpm…I still can’t get the engine to rev more than 4000rpm at about 25 knots.
Secondly…I can only back down to about 3300rpm before the boat falls back down off it’s plain. So the speed falls from about 20 knots to about 7 knots. I know that some of you might say…”change the impellor.” But in my opinion, that is only going to fix the rev position but not the drop off it’s plain. Yes??? No???

Even though the engine is designed to rev to 5500rpm, it sounds like it’s labouring to the max. I’m sure because 70 litres of fuel goes in 2 hours of driving.
I have now…AGAIN!!! blown a big end bearing & all the gudgens are over tight on the pistons. This has happened with only 6 hours on the engine.

The Jet is fully re-built by Doen. It has seen only about 12 hours of work & has chewed 4 engines.

I am quite disillusioned with Jets at the moment. I had high hopes that it would be the the ultimate fishing boat. But sadly now I think that Jet units might not suit this boat & might have to consider selling the jet & installing either:
a) a Bravo leg $$$$
b) a 200hp outboard on a pod
c) 2 x 100hp outboards on a pod

I’m hoping that some of you will have some great ideas before I start grinding out the ram tunnel. If you need more info or pics I’ll be glad to post them. Being the eternal optimist… I’m keeping my fingers crossed that I might be able to keep the Jet on.

This might not be a big thing for some of you but considering the fact that all I wanted to do is to go fishing 7 years ago and not to become a boat builder.

Hope you can help
Cheers

Did you contact Doen before you REALLY comitted to start this project? What did they say your hp - torque- cu. in. and rpm minimums would be? How close are you to the minimums?

[Sea Zar]

Ive been constantly discussing things with them and they have been very supportive. They did say that it sould be on the boarder line . They suggested re-boring the jet to an 8.5 inch impellor & maybe pitch down. This might help but costs another $2000. I would spend that if I knew that it would fix it. $2000 would be a good dent in either a stern leg or OB.Ill be on te phone to them today to give them specs on the cam as they asked for that. Ill keep you posted.