Prop Question from new member

[Paul F]

Hello All,

I found this site by chance recently, and was immediately amazed at the wealth of knowledge and information contained here.

I have a 2450 Command Bridge Bayliner, which did have the origional V8 Volvo / 275 sterndrive. I have now fitted an Isuzu 2.8 turbo / intercooled diesel ( 4JB1-TC ). The boat is now very economical for use in Norfolk, UK, our river speed limit is 6 MPH, but I feel that gear ratio and / or prop size is wrong. Currently I am using 15 X 13, 3 blade which allows 2800/2900 RPM.
The specs are:
Water line length 23'
Total Weight 5750 lbs ( 2600 kg )
V type planing hull
Engine 113 hp @ 3600 RPM
Existing gear ratio 1.61 : 1

Props for my drive are very difficult to obtain below 13" pitch, with much choice of 3 and 4 blades from 15" and above.
I could change the gear case of my drive to either 1.89 : 1, or 2.15 : 1

I would appreciate any advice re prop size and gear ratio. Regards, Paul

[baeckmo]

Welcome Paul, nice installation you have there! The performance of this Isuzu is very similar to the TAMD 30 engine (not the later TAMD 31), which was mated to the 290 SP and DP drive legs. It should run fine with the 1.61:1 gearing and even a 16" propeller on that boat.

There is a difference though in the exhaust channels of those drives, and I suspect that the reduced power you see in your installation may be due to high exhaust back pressure in the 275 unit, leading to low turbopressure and consequently reduced injection volumes due to the action of the smoke limiter.

Before taking to the big axe, you should be able to get information on the inlet pressures from Isuzu; check at full throttle with a good manometer (there is normally a threaded tap somewhere between turbo-compressor outlet and engine inlet). If charging pressure comes out too low, you have to redirect the exhaust, direcly through the transom via a low loss muffler (can be installed outside, below platform aft.

[Paul F]

Hello baeckmo,

Thanks for the information, you are right about the exhaust, the photo shows the initial installation last winter. I did sense there was a problem when first trying the engine. I was also worried that exhaust was not high enough above outside water level, so I changed it as these photos, with the outlet through the transom as you suggest. I did however do one part of it wrong, from the turbo I have 50mm pipe, with 75mm water jacket, where the cooling water mixes I reduced down to 50mm, and out through the transom in 50mm. At 2000RPM the 50mm outlet looks full of cooling water and is possibly causing back pressure, I should have kept to 75mm from the mix point, out through the transom. That is just one of the jobs for this winter

With the new instruments for this engine, I did install a manifold vaccum / pressure gauge. At 2800 RPM I just get 8 PSI positive pressure, at 1800 RPM almost zero pressure. I think this confirms that my exhaust is still restricting the full turbo boost.

In my main use of the boat on our rivers, at 6 MPH none of these things have any effect, but I would like to know that everything is correct for future use on the coast at higher speeds.

What do you think about the effect of using the DUO PROP drive with this engine ?

Regards, Paul

[baeckmo]

This engine needs 90 mm ID after the mix point and out. The primary gas pipe attached to the turbine outlet should be roughly 5 mm over the outlet dia; no further restrictions allowed. Absolute maximum back pressure is 1000 mm water column (~3 in Hg); aim for half of that.

8 psi, is that the inlet air pressure? If so, it is certainly too low, indicating a choked turbine outlet; rebuild as above. Even if you don't use full power, having a high xh back pressure will ruin your turbo unit.

The 275 drive is outdated (hydrodynamically and mechanically), the difference to a DP is quite radical, but of course it comes with a cost. The 290 SP and DP have wider xh channels and would easily accommodate the gas flow from your engine.

[Paul F]

Hello baeckmo,

Thanks again, perhaps I will go with 100mm after the mix, it's easier to get 100mm tube and bends. people with these engines in vehicles suggest performance is better with 2.5" exhaust from turbo, so perhaps I will re-make the whole exhaust from the turbo, first with 75mm, then 100mm at mix point, with minimal bends.

Re. drive, a company here in UK offer exchange lower units to change the ratio or exchange to duo prop with the choice ratios. the duo is of course a much hogher cost, then I have to buy props. I already have 15X13 and 15.25 X 15 single props, both new. People in USA with my model boat say change to dou prop is very good, but they are using petrol V8's.

Regards, Paul

[baeckmo]

With direct water cooling of the exhaust gases, some water will be evaporated. The piping has to accommodate the expansion of this steam fraction. At the same time the xh gases are reducing in volume, but there is a net volume increase (in part of the system length) that you do not have in a (dry) vehicle system. Hence the difference in diameters required.

If you go for a rebuild to DP, this engine should have the 2.30:1 gearing. Good luck!

[Paul F]

Originally Posted by baeckmo

If you go for a rebuild to DP, this engine should have the 2.30:1 gearing. Good luck!

Thanks for your help and advice. These jobs should keep me busy for some of the winter. I have to decide on 2.15:1 single prop or spend more money, and have the 2.30:1 duo prop

[andy2k100]

hi paul i have put same engine in isuzu 2.8 in alska 630 ,got a volvo 250 d raio leg think thats same as yours,17 pitch prop,boat 1500kgs prox ,give me 24knots max ,you are rite change the bottom gear box ,and tweet your power from 98hp to 120hp turn up diesel fuel presher 1/2 tern ,and fit water air charg cooler,ps my gasses go throw a 50mm out let and the leg ,with no probs,hope this helps andy