Sterndrive to Jetdrive conversion

[CarbonFootprint]

Hello,

I'm looking for some advice on jet-drives - I've been reading up on them for a while, looking at various installations and trawling through this forum, but I'm still a bit short on definite answers...

I have a 19' cabin cruiser (Shetland 570), planing hull, about 730KG displacement. Until now, it's been fitted with a 120HP stern-drive setup, but this has gone bang in no uncertain fashion and is beyond economic repair.

My options at this point are either to fit a new engine and stern-drive, cut the transom to accept an outboard or go for something completely different - i.e. a jet-drive. If anybody could point me in the direction of answers to the following, that would be most helpful;

I'm not sure what size jet is most suitable for my boat - from reading up, it seems to be about 6", but that seems quite small to me. All the jet-drive fitted boats I've looked at so far have been open, speedboat types - is my boat suitable for this type of drive?

Mounting the drive looks quite straight forward to me - I'm guessing it probably isn't, though. Can anybody give me a rough guide to fitting a jet-drive?

Mechanically, they look a lot simpler than a stern-drive - can anybody give me a rough guide to what overhaul and maintenance involves?

Any help and advice would be very gratefully received

Thanks,

Paul

[marshmat]

Hi Paul,

If you're planning to routinely cruise at high speeds, a jet pump might be worth considering. A relatively efficient commercial-grade pump such as a Hamilton 212/213 would be expensive, but might offer some benefits above 20-25 knots. Speedboat pumps like the Aggressor, American Turbine, etc. could also be used if you're planning higher speeds (say 30+ knots). Since I doubt your boat cruises this fast very often, I think I'd lean towards sterndrive options (or possibly an outboard), as these will likely be cheaper and more efficient.

[CarbonFootprint]

Thanks for the advice, Matt.

Paul

[baeckmo]

Concur with Marshmat's comments. Attached please find a graph where you can compare the statistically expected speed ranges as function of power-to-weight ratio. The diagram is a rearrangement of the trendline data from Otto Ranchi's propulsion sheets, together with some additional data from my own records (I hope this is OK with you Otto!!?). Note: power in hp, displacement ~weight in metric ton, speed in knots.

One propulsion arrangement that, in my opinion deserves more interest is the external tunnel with standard non-ventilating propeller. It must, however, be carefully designed, otherwise it will be unusable. Take a look at CDK's thread under "Surface propellers". Unfortunately, CDK had the design proportions wrong from the beginning, which are difficult to correct after installation, but his basic thinking is correct. In note #46 you find a rough scetch on what it should look like.

Anyway, taking this route would probably not be worth the effort for you, unless you already have a suitable reversing gear in your garage.

[anthony goodson]

A 6 inch jet is too small to perform well in your hull . If you are going to go jet, and as has been said it is not neccessarily the best option, look around for a PP/Vospower 90A. Make sure it's an A, this is a transom mount unit, a G will restrict your room in the boat as will most other makes As long as the ducting is not too corroded you have little to fear from buying secondhand and this should be cheaper than the equivalent Hamilton. Don't even think about an American style pump in this application ,the 90 has a 9 inch mixed flow impeller and was developed in a hull very similar to yours. As far as maintenance is concerned,very little is needed for a jet ,but don't get carried away by all this ,a modern outboard could still be your best bet.

[CarbonFootprint]

Thanks for the info, Baekmo - I'll certainly take a look at CDK's thread, but I think the sensible solution is to fit a new sterndrive...

Out of interest, I'm not sure how to read the graph you've posted, though...does it show an ideal speed for a given power to weight ratio? With a 130HP engine, the KR figure for my boat would be 17 - how does that relate to the trends shown?

Thanks,

Paul

[CarbonFootprint]

Thanks Anthony - the general gist of opinion here though, as you say, is that this isn't the best option. I was just testing the water (so to speak) with the idea...good job I did, by the looks of things.

Paul

[baeckmo]

Originally Posted by CarbonFootprint

........ I'm not sure how to read the graph you've posted, though...does it show an ideal speed for a given power to weight ratio? With a 130HP engine, the KR figure for my boat would be 17 - how does that relate to the trends shown?

Thanks,

Paul

Naaah, you got the math wrong. Previously you gave the figures 120 Hp and a boat weight of 750 kg. Add 250 kg's for crew and a beer, it makes 1.0 ton, then we write:

KR = 120^0.551 / 1^0.476; ie KR = 13.98, or close to 14.0. Find that figure on the vertical leg, go to the propulsion line of interest. With a submerged propeller max speed should be ~35 kn, while the SP prop would run in the 40 range, all provided your hull is reasonably designed.

But at anything like a sensible cruising speed (= a lower Hp/weight ratio), the std prop has the edge on efficiency. And, as always, Anthony makes a correct remark on impeller sizing (note that jet size should always refer to the impeller inlet diameter!).

[CarbonFootprint]

I see - my mistake - the old engine was a 130hp, not a 120hp. Is it all up weight rather than displacement that's important here, then?

So your graph suggests that there's really not much in it between a jet and a submerged prop for speeds over 20kts (one or two kts), but at half throttle on a 130hp, a jet might be 5kts or so down on a prop - interesting stuff, thanks for that.

Paul

[CDK]

Originally Posted by CarbonFootprint

Thanks for the info, Baekmo - I'll certainly take a look at CDK's thread, but I think the sensible solution is to fit a new sterndrive...

There's also this one, written before I started the tunnel drive project:
From sterndrives to jets to surface drives

No matter what jet you choose, to accommodate even the shortest universal joint the engine has to move forward approx 12", much more if the jet intake also must be located in the boat. Lots of alterations, unforeseen mechanical issues and an unhappy end.