How much power

Thanks for the prompt reply. I will dig through pics andt drawings and post but it tapers from 4.5' beam to 0 in 6 ft. ( takes 6 feet to go from 27" to 0)

 

Okay, you built her. Now that you have her, please provide the propulsion system details - engine model, tranny gearing, shaft size and length, bearing number type and location, and prop pitch and diameter. Then post an rpm vs speed graph for calm seas, and also for into wind and seas if possible. And we need the final displacement.

The prop problem really needs to be fixed. I doubt the speed issue is going to change, though. It looks like a 9 knot boat to my eye. You can't put a tuck in the stern like that and expect the water to follow at any speed over a few knots. The easy (sorta easy) solution would be to put a 3' longer shaft in her and add a skeg. Increase the shaft diameter as needed to handle the extra length. Position the prop at about 96-98% of the waterline. Trim the box keel bottom upwards at about 5 degrees from running trim starting about 60% of waterline.

Your fuel cost is going to be several times what an efficient, plain vanilla monohull would manage at speeds over 10 knots.

Cutting the amas off, filling in the stern step/box keel thing, and adding a couple tons of ballast in an external keel would probably get you some speed back.

 

Allowable tapers are a function of speed. For planing craft that aren't too concerned about fuel economy, bottom tapers of about seven degrees are normal (ie, the max that can be tolerated) but understand that the boat will trim bow up at speed, reducing that rise angle as the boat gets on a plane. Side taper on fast boats is usually restricted to about 3 degrees and it needs to be developed gradually with a large radius. Fully developed foil sections can get away with a bit more than 3 degrees. That's why transoms of fast hulls tend to be broad and shallow - it's easier to bring the bottom up than to bring the sides in.

 

Thanks Phil,
Bringing the bottom up would be easier. I dont have much data yet to provide the graph but will provide soonest I can get them. So this is a Mitsubishi 6d14 (2800 RPM) with a Hurth HSW630 2.1 ratio on a 2 inch shaft 8' length with 3 wooden bearings. Prop is a 24" dia with a 17.5" pitch. My suspicion is the blade size of prop is too small creating the cavitation.

 

Empirical knowledge is very valuable and effective. However, you have to listen to those that know a lot more than you and have experience. Changes are very gradual in traditional boatbuilding. Also, the designs are very specialized for their intended use at a particular location. You are dismissing engineers and naval architects, while at the same time dismissing shipwrights. Arrogance and an aptitude for insults does not make you an expert in boat design and construction. Being humble in front of those with a large volume of information will serve you better to learn.

 

Standman, re your cavitation, the cause might be a lot more basic than blade area ratio of your propeller - it might well be that the propeller is simply not getting enough flow coming to it, because of the relatively abrupt change of hull section further forward.
Is there any way that you can improve the flow into the propeller?

Many years ago I worked in a yard building GRP 27' semi displacement fishing boats - they were fitted with John Deere 4045TFM engines (I think about 140 hp), and they worked very well.
We were asked to build a dive boat on the same hull, but with a John Deere 6o68TFM 220 hp engine.
These boats all had 4 blade propellers.
We took the dive boat out on sea trials when she was launched and although we were hoping for 20 knots, we couldn't get over 10 knots because the propeller started cavitating terribly. The propeller was the first suspect to blame, but eventually the problem was cured by simply fairing the keel deadwood forward of the propeller shaft log a bit to improve water flow to the propeller.

You mention that your Mitsubishi should be capable of achieving 2,800 rpm; what speed is it running at when cavitation starts?

 

Hi Bajan, I notice it around 8 knots. I will try to have a lower pitch prop ( go from 17.5 to 15.5) then fair keel deadwood . more than 70% of the prop is below keel deadwood and not affected by water flow. Is it possible the approx. 30% causing cavitation?

 

If the deadwood is not faired properly it will cavitate not the propeller. The propeller blades may be operating on the steam bubbles created by the deadwood.

 

It does seem that turbulent, stagnated water over at least part of your propeller disc, would be an open invitation to cavitation. And if it resisted cavitation, would be working overtime in a water flow that simply was not flowing freely into the prop. As Phil mentioned, you could get away with that sudden tuck-in if travelling at crawl speeds, but the flow will just separate and eddy at a relatively slow speed. You are getting the double whammy, the "suck" at the stern, plus unsuitable conditions for the prop, resulting in disappointment. I am inclined to think that if I attempted a fix, it would be to double the length of the taper, but I don't know how much room you have to play with.

 

How did you determined the problem is cavitation and not ventilation?

 

Mr. E,
I believe this is what you mean. Should I try first a lower pitch prop or would that be futile ( before i modify the hull)?
Gonzo, I'm not 100% sure it is cavitation as oppose to ventilation but since i dont think i really have much air induced, i am leaning towards cavitation.

 

You appear to need something that more mimics a fish shape, which can taper in both the horizontal and vertical plane, a lot depends on your drive, it has to be compatible with whatever changes you make. I think if any part of your prop is behind a blunt taper, it is no good regardless of the pitch. Is there nothing protecting the prop from grounding ? I would have thought a prop behind a long horizontal taper, with little or no shaft angle, would have solved the cavitation and drag issues, as well as protecting the propellor, but the dimensions maybe don't allow it.

 

Ilan Voyager 21.8 meters, 7 tons fully loaded, 28 knots with 240 HP.

 

Beautiful boat. Was what I looked up to and served as inspiration.