Calculating hull form constant for C-Dory 25

Good Day Eric! Interesting site particuarly with some of the talk on what some call semi-displacement hull types.

Back to your prop pic...and repower. I guess we need to figure out which motor you go with first due to gear ratios and other factors being different on the motors these days. Figure you are going 4 stroke....

Then, looking at your post a bit... seeing the 6 week trip last summer on your boat, you may want at least two props. One for "light loads"...and another different pitch for "heavy/cruise loaded" loads. Sorry about the double load there....but sort of fits the topic.

I know several of our 25 owners have tried several prop set ups... some better than others. One thing that does seem constant, is those who mount a Permatrim gizmo....sure are glad they did...but then....that in itself is an entirely different link....and has also been beat to death and brought back to life a time or two....

I always have enjoyed owning two props for each motor... One for light, one for heavy...and only takes about 10-15 minutes to swap out, but, go ahead and buy the extra small parts that will fall thru your fingers when they are in the cold water.....or, deep water changing the prop. I mean, who really changes the props while we have the boat on the trailer on dry land anymore :mrgreen:

Hope to travel in the NE a bit more in 09. Last Dance (Terry) brought me bye the canals a few times by road in 08....just to wet my wondering lust.

Good luck.

Byrdman
 
If you know the weight, which is somewhat variable, I used 6000 for a guess.
And know the top speed, which is very variable and based on conditions. I used 28 and rippled water, best possible conditions.
And use my existing HP, which is a known, 150HP, I get a hull factor of 177.1.
You can compare that to the scale and see where you fit in. However the top speed is dependent on the prop specs and the water conditions. If at WOT you are at the recommended rpms then you would have a meaningful number for the hull factor.
But I am not at all sure what knowing the hull factor gets you. How does it help with prop selection. Or am I missing something here.
 
We are going from a Suzuki DF 140 to a Suzuki DF 200. I scanned through the CD-25's listed under "our C-Dorys" and didn't see any one running the DF 200.
The rigging should be straightforward for the motor but we could find little information about prop selection. I asked Ken at propgods for advice and part of the information he wanted was the hull constant calculation - when I find out how that helps him I will post it here.
As Patrick noted we do run heavy and with the 140 had to use a 3-13.5-15 prop to get to get around 6000 rpm at WOT.
It would be great if a few more people could run their numbers on the hull constant calculator and I will send the numbers of to Ken.
http://continuouswave.com/cgi-bin/crouchcalc.pl
Thanks
Eric
 
Eric using 3225 pounds as a weight for my 22 with my 90HP Suzuki my hull factor comes to 173.6. I think this weight is pretty close and my top speed is 29 mph. I'm still scratching my head about what use this is. I guess you can determine top speed for a repower once you know hull factor.
 
Eric

what is you repower motor of choice.

there could be about 2 dozen folks with that motor and the same boat who may be able to get some no "shuks" this is what I have tried....in the water loaded...

Just a thought...but, playing with numbers is fun too.

Byrdman
 
I don't know of anyone with the 200 either on the 25. I talked with Marc at Wefings about a 175 Suzuki (remember that the Suzuki's use a different gear ratio than many other engines do)--and he suggested a 19.5 x 16 (the 200's and 175/150's use the same lower unit and props).

You are going to a much more powerful engine--and one which will push your boat over 30 knots. I personally didn't think it was worth going to the 200, but it would depend on how the boat is loaded (we run pretty well loaded currently with a 130 Honda, which is a bit less powerful than the Suzuki 140.

The problem with these prop calculators, is that there are so many factors, and I believe in this case you would be seriously mislead by the calculations. The lower unit gear ratio is very important, as is the diameter. You want to get full ROM (6000 in this engine) at light full speed or Wide open throttle. Also is how the boat is loaded, what the trim of the boat is, elevation, bottom condition etc.

Best to try different props--or talk to Power Tech--they are probably the most knowledgable. I would also ordinarily agree that two props is a good idea--and it is for nothing but back up--but in this case you will be putting on an engine at the max rated for the boat and it will still perform well at 3700 foot altitude. If going to 7000 feet, then consider one prop at least 2 to 3" lower pitch.

A dealer with a number of props to try is the best way to go! Not the calculator.
 
One thing is definitely wrong with the logic here. That is, the hull factor is being used to determine the type of prop. BUT the top speed is part of the hull factor calculation which in turn is dependent on the type of prop in use. So, I can't imagine that the hull factor numbers will be that useful in determining what prop to use since the prop that was used in determining the hull factor to begin with was not part of the calculation. So, my bet is that in the end, you'll wind up doing what everybody else does, ask someone who is close in horsepower, take a good guess and play with 2-4 props until you're happy.
 
Type of prop has nothing to do with the calculation. The calculation is useful if you have weitht, HP, and top speed. That will give you hull factor. Then, you can change the HP (or weight or both) to find out new top speed, if you change nothing else, or enter a top speed to find out how much HP you need to add or weight you need to shed to get there (top speed) changing nothing else.

If you change props, start all over again, and remember, it's all just an estimate!

Charlie
 
I think one thing it's good for is determining how effecient your prop is using this tool http://www.propgods.com/content/prop_calculators.aspx. In my case my current prop has 15.25 pitch if I plug in the other numbers it gives me a prop slip of .05 which I guess is pretty good. This is with a permatrim and the motor raised one hole. Wish I had run the numbers before adding the permatrim and raising the motor. I'm going to add some cupping to this prop as my motor revs are a little to high and rerun the numbers.
 
Helm wants to use this formula to determine prop pitch as per his first post. The formula does not deal with prop pitch. The C Dories generally behave well up to about 30 hp. In my (and others) experience the boats do not do that well after the low 30's. It is not a fast boat, nor is it designed to be a fast boat. There are multiple factors--and a number of these are not address in this formula--which determine the top speed. For example, the small keel, the reverse chine, the amount of rocker, the flatness of the run aft.

I submit that this formula is of little value in ansering the question which Helm asks. Proping a boat correctly is often trial and error--because there are many variables. There are people who are experts--such as Power Tech and several other prop providers who have a lot of experience. Same for the engine manufacturers. Use their advice--not some formula which comes off a Boston Whaler owner's site for determining the prop pitch.
 
Captains Cat":32hdlhnv said:
Type of prop has nothing to do with the calculation.
Click to expand...
That's my point exactly. Since top speed is dependent on the type of prop you have, the "hull factor" by that formula HAS to include a contribution that is specific to the type of prop on the boat used to determine the max speed. That is, this formula to calculate the "hull factor" actually calculates something that is a combination of hull and prop.
Captains Cat":32hdlhnv said:
The calculation is useful if you have weitht, HP, and top speed. That will give you hull factor. Then, you can change the HP (or weight or both) to find out new top speed, if you change nothing else, or enter a top speed to find out how much HP you need to add or weight you need to shed to get there (top speed) changing nothing else.

If you change props, start all over again, and remember, it's all just an estimate!

Charlie
Click to expand...

I agree that the formula will likely give you an estimate of the new top speed if you change horse power or weight. However, the idea that this estimated "hull factor" is particularly useful in determining the prop to use on a different horsepower seems a bit off base to me since the prop that was (indirectly) incorporated into the original estimate of the "hull factor" is not part of the formula. Doesn't make physical sense to this scientist.
 
I believe Rogerbum is exactly right. The only way this makes sense to me is if the "hull factor" is more like a combination of weight, design and theoretical speed (cruise? max? desired?). Using actual speed produced by one prop to calculate a "hull factor" for choosing another prop does not make sense. Put another way, "hull factor" implies something more or less immutable about the boat. Including the engine in the calculation seems equally hinky. Allowing the changes in the prop to change the "hull factor" really takes us in a meaningless circle.

Start with the experience of others, and then use trial and error.
 
Try looking at it this way. Keep the HP and weight constant, record the top speed. Get a hull factor.

Change the prop, keep the HP and weight constant. The top speed will change, and, if the formula is correct, the hull factor should be the same as before because the "hull" has not changed. If it isn't, then the formula is not correct.

The hull factor is the hull factor is the hull factor. The only variables in that formula are the HP, the weight and the top speed.

Charlie
 
Maybe it's just semantics? I think it's a little deeper than that, but whatever . . . I believe there may be a more accurate description of the "factor" we're dealing with. If the hull, weight and engine do not vary, what we are calculating is the effect of changing the prop. That would be the "prop factor."
 
Captains Cat":8jxb47jb said:
Try looking at it this way. Keep the HP and weight constant, record the top speed. Get a hull factor.

Change the prop, keep the HP and weight constant. The top speed will change, and, if the formula is correct, the hull factor should be the same as before because the "hull" has not changed. If it isn't, then the formula is not correct.
Charlie
Click to expand...
Check it. The formula is not correct since it does not include the prop.

Captains Cat":8jxb47jb said:
The hull factor is the hull factor is the hull factor.
Click to expand...
Yep. If the hull factor is truly a measure of the efficiency of the hull and not something that is calculated from ANY formula using only HP, top speed and weight UNLESS there is an implicit assumption that the prop is 100% efficient (or some other fixed efficiency) in transferring the horsepower to the forward motion of the boat.

Captains Cat":8jxb47jb said:
The only variables in that formula are the HP, the weight and the top speed.

Charlie
Click to expand...
That's the problem with that formula. Trust me on this Charlie. My point is that the only way that formula makes sense is if it assumes some fixed efficiency of the prop. However, since the intended use of that formula is to pick a prop AND since the "hull factors" are being calculated from props of unknown efficiency, the intended use is incompatible with the estimate. In fact, since Marvin used the hull factor and some other formula to determine that his prop slip is 0.05, I'd bet that the hull factor formula assumes 95% efficiency in the prop.
 
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