Calculating hull form constant for C-Dory 25

Actually I'm pretty confused still about what use the hull factor figure is. Yesterday when I was testing I recorded my MPH and fuel flow at RPMS from 2000 to 5500 every 500 rpms. My results at 4500 and 20 mph was 4.83 gallons per hour or 4.14 mpg. When I plug those numbers in to the formula it gives me a prop slip number of .02. At max RPM of 5500 I had 29 MPH with a prop slip of .05 and gals per hour of 7.79 and 3.59 mpg. This doesn't really address the hull factor but it seems to prove the prop slip formula is somewhat accurate.
 
I'm so staying out of this one. But I did ride on a Cape cruiser with a 200ph e-tec and it was really nice. I don’t know what his fuel burn was a WOT but the boat was able to scoot right along at 30mph with the bow down, which made for a better ride. At 22-25 mph the motor was just purring along. I have always felt that 135 to 150 hp were too little for the c-25. Also as long as we are talking about variables of engines let’s talk weight. As an example--- Honda 135 weights 485lbs while the Suzuki 140hp weights only 410 and the e-tec 200 weights 433, less than the smaller motors. Hell a 200 Honda weights 599lbs. 166 pounds more than the e-tec. My point is when you are figuring more hp you have to have the right weight, not a static weight regardless of the h.p.

Now correct me if I'm wrong but don’t boats have a Hull factor at which point more hp. only burns fuel and does not generate more speed? Or is that only sail boats?
 
starcrafttom":wwvolwyq said:
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Now correct me if I'm wrong but don’t boats have a Hull factor at which point more hp. only burns fuel and does not generate more speed? Or is that only sail boats?
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That's only for displacement boats (a category into which most sailboats fits). For planing boats, more horsepower generally gives and increase in speed, the issue then becomes handling. With a C-Dory (and this is Bob's point), the hull doesn't handle well at above about 30kts so one could argue that the max horsepower that makes sense is the horsepower necessary to get to 30kts.
 
starcrafttom":1smq99k2 said:
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Now correct me if I'm wrong but don’t boats have a Hull factor at which point more hp. only burns fuel and does not generate more speed? Or is that only sail boats?
Click to expand...

The above is the generally agreed upon principle for displacement hulls, like heavy sailboats, trawlers, tugs, etc., and in fact, pretty much all boats except planing hulls and hydrofoils.

A displacement boat is trapped between it's bow wake and it's stern wake, and applying more power just makes the boat generate bigger waves fore and particularly aft, dragging a big stern wake along.

Generally, planing hulls have to be light in weight, have flatter bottoms, sharper chines, and enough horsepower to enable them to climb up onto and even over their bow waves, generating instead a wave from their flattened aft sections of their hulls as they plane along on the surface of the water. (This statement is a generalization and there are various exceptions to parts of it for some types of modified hull types and semi-displacement or semi-planing hulls.)

The formula for displacement hull speed (a generalization, again) is:

the square root of water line length times 1.34 = speed in knots,

and the maximum hull speed of a boat therefore goes up with the it's hull length accordingly, and longer boats are therefore inherently faster, all other factors considered constant.

Hope this is clear and straight-forward enough to be useful!

Joe. :teeth :thup
 
Eric, I have a 200 hp etec on Shearwater and looked at the Suzuki also.

I believe that etec has a slightly higher final drive ratio (numerically lower) than the suzuki. There are two schools of thought on this
1) you can swing a prop with a larger pitch or...
2) it helps to make up for the reduced torque of a 4-stroke at lower RPM VS a 2-stroke

Depending on which one of the above you buy into and for all it worth

I am running a 15X17 aluminum. 5400 RPM max and a little over 36 MPH at top end. I would estimate total weight around 5500 lbs when I got these numbers and feel that I am a little over propped when loaded. The etec has a max RPM of 5800 and 5600 is optimal
Also have a 15x15 SS that I have not had on the water yet but doubt if I see much of a change and may have it taken down to a 14 pitch.

Might give you a starting point

Been very impressed with my etec and weight on transom is about equal to a 150 4-stroke

stevej
 
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