Power options for jet conversion

Have been working on a 17.5' fiberglass hull that was originally powered by a 4cyl OMC (120HP???) driving an OMC outdrive. I have removed the deep-vee and flattened the aft 1/3 leaving a lifting strake on each side, the forward 2/3rds has a shallow vee but the deep fore foot is gone. The hull without motor or pump weighs about 2050 lbs. I will be using a Hamilton 751 pump with standard impellor and am planning to marry that to the original OMC 120 HP. Question, what kind of performance do you think it will give me. Primary use will be on Pacific NW estuaries, the Columbia River and inland lakes, 2-3 people on board, no white water. I have not mounted the motor yet so now is decision time.

 

Estimated displacement 3'000 lbs abt. abt.
Hull + wj + engine + cooling liquid + lub.oil + security equipment + gasoline + 3 men.

Here you have a graph from Hamilton to check rushly the speed.

 

I'm waiting your comments.....(if any)

 

I appreciate your comments, am digesting what you said. That table seems to be for commercial applications & the boat length is in meters. The smallest length boat is for 6 meters (25'), mine is 18', using my single stage pump, I was hoping for practical experience with similar application.

 

Hello Mr. OTTO,

we don't all read graphs or grasp math. as quickly as you do. That includs me.
So give us a little time. I appreciate your generousity of last week and
I'm still absorbing it as well. - ManyThanks !

To twisterme,

Mr. OTTO's information is sending you in the right direction.
As Mr. Hamilton was charging up rivers in 18' jet boats, before most of
us were born.
Since he passed on - the company has become - shall we say - more
corporate ? However the basic solid work he did with his jet still works.

For good or bad - however, Hamilton was never shy to overcome
problems, with more power. Like 250 to 300 hp in 18 ft of river boat!

So the number one thing to do - is don't be under powered !
Mr. OTTO is giving you a hint.
The real number it's best to consider is that, including everything:
3000 lbs is the effective, real weight - to anticipate.

Rule of thumb; is a planning boat needs a power to weight ratio of:
15 to 1 to 10 to 1. Jet or prop. Even if you intend to keep your speed down.
This is basic. So the power rating you quote: is a bit on the skimpy side....Sorry.

Roughly speaking, it takes twice as much power to get on plane - as a planning
boat needs to maintain a given cruising speed.
Therefore; even if 100 hp is enough to cruise. You might need 200 hp to get there.

Also, I've said it before - and I will say it again.
Propellers are somewhat imprecise. And you can mix and match - a wide
varietry of props and engines. And the boat will still mosey along.

However...........!
Even a well designed jet and boat combination will be most efficient, only in a
relatively narrow band.
Above and below this power/speed band; efficiency can drop off. A lot !

There are all sort's of varibles here and it's hard to be specific.
Basicly, most of the time - you end up having to chose between
low end thrust. Or good top end. But you can't have both.

I've seen backyard jet boats - that came in too heavy
- that were a serious disapointment.
Everybodys, always suprised how heavy their boat comes in.
It's a rare vessel that hits the water - below - the calculated weight.

So I've heard of relatively, fast jet boats that would get on plane with 2 pass.
- but could not with 3 ! Too many stories, like this....

Another however...However...

There are lots of jet boat guys in Washington & Oregon. Many, only a local call
away - I'm sure. Who can give you some hand's on advice.

Good luck and fair winds !

 

tri-star
thanks for your info, one of my local friends (jet boater) was basically saying the same thing, thats why I thought it best to get some more opinions before I commit to a motor..... note: this pump had been installed in a local law enforcement boat boat behind a Volvo 4cyl........thanks again