Keel modifications

[baeckmo]

Rammy: first, as Ad Hoc has remarked, there is something that doesn't add up in your rig here. A preliminary calc on your propeller shows a remarkably low efficiency, further reduced by cavitation due to high load.

1/ There are a few versions of the Velvet V drives that do not have the same ratio fw and reverse. There is one with 1.21 fw and 1.33 rev f.ex. Fw direction should be cc-wise (if I recall correctly...). Please check prop shaft rpm,s with a tacho!

2/ There are versions of the Perkins 6354 with max 2400 or 2600 rpm. With hot engine and in neutral gear, check maximum no-load rpms (increase throttle gradually!), that might indicate which setting you have.

Unfortunately, I can not see the propeller and its "environment" in the pic; there may be a wide chunk of keel destroying inflow to prop as well. It's a nice boat you have got there, and she certainly should have better performance than 8 knots. If you could provide answers to my quests above, we should be able to find a solution, so don't rush away bying a new screw yet!!

[Rick Willoughby]

Originally Posted by RAMMYBOY

Hi Rick, The shaft rpm are 2314 according to Castle marines calculation. There was no serial number on the gearbox (borg-warner velvet drive, on a vee-drive) I calculated the ratio with an optical tachometer on the shaft and my new vetus dashboard tachometer. The nearest ratio in the borg-warner book was 1.21 . I have tried different prop diameters on Castles site and they all say that the ratio is not ideal,although they do not tell me the ideal ratio. It looks as though I may have to alter or replace gearbox. What do you think? My main concern at the moment is deciding on the approx diameter of ideal prop 16" 17" 18" 20"? so I can work out new stern tube position. Thanks for your interest. Rammy
P.S. my taco shows 2800 rpm

Rammy
You might find this thread of interest:
Hull modifications

It is not the same as what you are doing but the boat is a somewhat scaled up version of your Aquabell. Blizzard has the advantage of shallow tunnels so it can swing big props without going really steep on the shaft.

The main thing to note is the size of the props and the weight of the boat. Roughly twice the weight and two big props that achieve good efficiency. Drag on this boat is less than half that on the Aquabell so prop loading is much much lower.

Your 14" prop will achieve around 40% efficiency. At 12kts you will be getting toward the drag hump that is not much less than what would need to be overcome to get fully on the plane although you will have nowhere near enough power to do that. However your prop still has to generate roughly the same thrust as that required by your hull to plane. A single 14" prop is a very inefficient means to achieve that - 60% of the fuel you are burning is simply churning water.

The other thing to note is the performance gain on Blizzard by cleaning up the underwater bits. If your long keel is poorly streamlined it will add significant losses.

Rick W

[Ad Hoc]

I'd be interested in your opinion baeckmo.
From the pic's, i'd say he's got a wake of around 10%-ish. (maybe ~15% with the size of the skeg)

But the speed of the boat i reckon is not wholly down to the high loading on the small prop, (hence a larger prop is required)... the characteristics of the hull does not lead itself for big gains, just small ones. The form factors are not ideal.

Having said that, I think the reduction ratios posted of 1.99 or even 2.5 would improve matters considerably, rather than just changing the prop diameter. A combination of the two are required. This of course would lead to a much larger prop diameter than originally envisaged!

[baeckmo]

Yes, Ad Hoc, I certainly agree on your reflections regarding gear ratio and wake (hence my note on picture). Using a wakefactor 0.15, a shaft depth of 0.7 m, shaftspeed 2314 rpm and calculating for Wageningen B 4 blade with BAR 0.69, the noncavitatingresult is:

Ja=0.26, eta=30%, Pprop=136 hp, T=8458 N, Sigma07=0.16.

In order to limit cavitation to ~10% of the blade area, a BAR of 1.83 would be required (30% cav needs BAR 1.13)!

The available shaft power is ~110 hp, provided the engine is healthy and drinking good diesel fuel. The sc "green diesel" or "envirodiesel" or whatever is reducing engine output power some further 8%. Checking a "generic" engine power characteristics, this engine would have managed ~2640 rpm with this prop, if it were operating cavitation-free. Since the owner reports 2800 rpm, my conclusion is that it may be working on the governor rpm limit while heavily cavitating.

Obviously, the thrust is seriously reduced. I won't spend time to dive into detailed calc on cavitating performance now, but just using a "tau/sigma"- rule-of-thumb diagram, I would not be surprised if the effective thrust were down in the 5100 N region. First step is thus to reduce blade speed/increase disc area.

One more indicator for propeller load: the Taylor Bp factor would be over 200 if the full herd of horses were kicking around. This simply can't work! The honoured sir Charles Algernon Parsons and his tandem propellers comes to my mind..........!

Now I would like to have a response to my previous notes before we start any serious brainstorming here!

Come to think of it: It might help in the selection if you Rammy could provide a set of speed/rpm observations; say from 5 knots up in steps of 200 rpm.

[RAMMYBOY]

Thank you all for your interest, the hull modification thread makes my problems look minor in comparison! I will check the engine RPM again, pretty scary being that near to the old girl in full flight!! I will check the shaft RPM again(even scarier with head under the deck!) also check reverse ratio. I will get some pictures together of boat static, underway and stern gear. I am not looking for a quick fix, so won"t be rushing out to buy a new prop just yet! Plan to carry out this project in the spring, so am glad I started the research in plenty of time. Rammy

[Ad Hoc]

Rammmyboy
"..I am not looking for a quick fix,.."

Perfect. That is the most sensible retort anyone can give. Problems such as yours requires more than a one liner and a pretty picture reply from anyone. So I'm pleased to read this.

Baeckmo will have far more data at his fingers tips than I. (I have to wipe the dust of my old charts!!). So, once you're able to overcome your scary adventure , I'm sure we can get to the bottom of this. Just takes patience, which it appears you have plenty of