going slow with jets

[panterapaul]

hi iam new at posting but have been reading posts here few years now . very informative.my question a bit unusual . i have a 35 ft moderate v planing boat ,currently using two 350 mercruisers .the boat is very light only weighing 12000 lbs .i would like to change to diesel power with two jetdrives. I have read lots in this forum about efficency of jets at low speeds but thats of little concern to me as these days i mostley sit on the boat in a marina, enjoying the sunsets. I do not want to go past hull speed so i thought two 70 hp diesels would suffice . i like twins because of the fantastic docking ability of twin jets . So can you guys help me in choosing a jet . Two other resons i what to fit jets is i dont want loose my midcabin and also i go fishing in very shallow water . Any help very much appreciated .

[whitepointer23]

give hamilton jets a ring, they will work out the best solution for you. they have agents all around aus.

[Submarine Tom]

My thoughts exactly.

-Tom

[CDK]

If you intend to stay below hull speed, 40 HP @ 3500 rpm engines are sufficient.

Changing from stern drives to jets is quite an operation. I understand the desire to get rid of the drives, but have you considered other options?

[mydauphin]

I would go with one outboard. You gain space, you can tilt it out of water, and get right power level. It will be a lot lighter too. Get an outboard with a big prop, I believe Yamaha has a few models.

[Stumble]

My advise would be to trade in the boat on something that was designed to do what you now want it for. The design compromises to get a boat up on a plane are pretty severe, so why pay them if you won't take advantage.

[panterapaul]

thanks alot for the imput i will take it all on board try decide best way forward

[joepaiva1]

This may sound crazy, but...

There is a 17' Yamaha jetboat for sale in my area. The hull, upholstery, trailer are all in good shape, and the jet drives appear to be fine. However, the power plants are completely gone.

I was planning to start building a 4 passenger, electric powered boat for cruising around the flat water at 5-10 knots. However, at $1,500-$2,000, the price of this boat is less than what the marine plywood, fiberglass and parts would cost to build a 17" boat and trailer.

So... I am thinking about buying the hull and either replacing the jetdrives with props, or, running the jet drives at 3000rpm (which would push the boat at about 5.4mph assuming the displacement doesn't change).

What do you think? Should I try the conversion or just start from scratch with a homemade frame and stitch and glue?

Thanks in advance for any advice.

Joe P.

[Stumble]

I am not usually a fan of conversions from one type of drive to another, but in this case it might work. just cut everything out and install a torquido electric outboard on the back... the loads should be so low it almost doesnt matter where, and go enjoy.

The boat will perform poorly relative to a boat designed for these speeds, but at he price...

[sottorf]

Originally Posted by panterapaul

hi iam new at posting but have been reading posts here few years now . very informative.my question a bit unusual . i have a 35 ft moderate v planing boat ,currently using two 350 mercruisers .the boat is very light only weighing 12000 lbs .i would like to change to diesel power with two jetdrives. I have read lots in this forum about efficency of jets at low speeds but thats of little concern to me as these days i mostley sit on the boat in a marina, enjoying the sunsets. I do not want to go past hull speed so i thought two 70 hp diesels would suffice . i like twins because of the fantastic docking ability of twin jets . So can you guys help me in choosing a jet . Two other resons i what to fit jets is i dont want loose my midcabin and also i go fishing in very shallow water . Any help very much appreciated .

For the size of jets you are looking at, you can also consider thefollowing jets:
1. Alamarin (Finland) - great small jets
2. Doen (Australia) - very good prices, very helpful guys
3. Castoldi (Italy) - jets with reduction gears integrated.

I know Alamarin and Doen will work with you to choose, engines and jets to suite your application. You could go with a small Steyr or Yanmar diesel.

[Ilan Voyager]

Installing jet drives on a boat not designed for that purpose is quite a big job: you have to change all the disposition of the machinery and KEEP THE SAME CENTER OF GRAVITY. Not all hulls are usable for that. Truly a big big job. Very expensive also; a Castoldi for a 365 HP diesel on a 30 knots boat cost only 26000 Euros (around 35000 bucks) plus the installation on the boat, that adds easily 25000 bucks more. It begins to hurt...
Smaller do not signifies more cheaper. It means only slightly cheaper. A planing boat even light at hull speed is at the worst drag; just on the big hump before planing...A planing boat must plane because the required power for keep planing at let's say 18 knots is far less than the power required for a constant 13-15 knots. And moving a planing boat at 10 knots is not very efficient with the huge wave behind. A boat is always optimized for a certain speed. That explains the different hull shapes...

Besides the price, the jet drives are not very efficient compared with a good big propeller, except on boats specifically designed for. The domain where jet drives are worth is very restraint.

Maybe you should consider if feasible without too much work is to use only one engine, like a 5.7L or 6.2L Mercruiser fresh cooled water with a Bravo II with a big 20 inches four blades propeller or a Bravo III. The efficiency of these engines with a Bravo II is surprisingly good; the boat alleviated of the weight of one engine and its gas will probably run with a better efficiency (ie low consumption) with a reasonable loss of speed. Maintenance is cheaper also. You can have for security a 60 HP outboard on a bracket in case of problems with the main engine. A 6.2L with a Bravo II is dirt cheap compared with a 70-100 HP diesel with a jet drive optimized for "low" speed.

The Mercruiser diesels seem to have reliability problems (run too fast for my taste...) and it seems that they have not a very long life expectancy. I say seem because I have only second hand information. Furthermore going to diesel is expensive and only valuable is you use the boat at least 500 hours/year. A diesel used only 100-150 hors/year is prone to corrosion problems as the combustion of diesel produces some sulfuric acid. You can expect the premature death of the exhaust valves unless a very careful maintenance.