How many pounds of force to reach hull speed

[Franklin]

Curious what it takes to get my boat, Hunter 376 with 18,000 displacement (including all the stuff on it) and 32' of waterline, to the 7.6 knots of hull speed. Any way to determine this assuming all things such as bottom and prop and everything is in perfect condition? I'm just looking for a rough estimate.

 

[Franklin]

oh yeah....and fixed prop

 

[kloudie1]

Roger Long probably has some rule of thumb that will get close.. This was discussed a lot sometime back when we were talking about electric power systems for propulsion. One of the guys may have posted a guesstimator ..

 

[Napa Ed]

pull it behind a bayliner with a fish scale on the tow line

 

[tsmwebb]

Mostly just a guess, but I think the answer will be somewhere around 600 pounds... But, keep in mind that the drag curve goes exponential so small changes in speed make for big changes in drag over fn ~.4 (s/l of ~1.3). Adding a fraction of a knot will push the drag over 800 pounds, dropping a fraction might put you down into the 300 pound range... Generally, to get a good answer to this you need to calculate friction, roughness, pressure, wave, heal, induced and added wave drags and then sum them up... At the speed you're looking at the vast majority of the drag is wave drag. This is often calculated using an interpolator for one of the "Delft" parent models. Methods are available in all decent yacht design books. I like Larsson & Eliasson's Principles of Yacht Design. You'll may well need to pay for a good answer since the fun part of yacht design is the preliminary drawings. Everything else is just work.

--Tom.

 

[kloudie1]

Here is a link to the question back from January when we were discussing electrics replacing diesels.. http://www.proboat-digital.com/proboat/20070607/

 

[Don12364]

With standard 2 blade prop and 7.5 kt boat speed - about 400 lb.
Bollard pull stationary approx 750 lb.

Remember this push/pull is added to the torque and then transmitted up the propshaft and into the engine mountings.

And we all worry about getting the flanges aligned to 0.004" or better!

 

[Franklin]

Most variables are not important for my goal. Was curious how much force I'm putting on my one stern dockline tomorrow when I put it in gear and floor it and then go down below to check to see if I can manually increase the RPMs at the engine (testing the linkage because not getting more then about 2200 RPMs). I know my engine can reach near 7.2 knots so I guess I'll be putting on about 200 lbs pull and while I'm down below, if I can increase the RPMs, then maybe up to 400 lbs. Cool. My 5/8 dock line can hold it without a problem and I know my cleat can.

 

[Franklin]

I was worried it would be up near the 1,000+ lb range. I know I've done this before but never with me leaving the helm. I always feel nervous when boat is under power or sail and I leave the helm; even when I'm out in the middle of the gulf with nothing around.

 

[tsmwebb]

Most variables are not important for my goal. Was curious how much force I'm putting on my one stern dockline ...

Oh, then you can ignore my response as I was calculating hull drag at hull speed.... Tied to the dock that's going to be zero drag at zero knots...

Donalex seems to have answered your question. I'm not quite sure how, since I'd need to know something about your prop and motor to even make an estimate... But, the number you need is the bollard pull of your boat.

As a safety thing, it might be wise to conscript somebody to stand by the controls in case you rip a cleat out of the dock or something... Most marina's frown on this kind of testing for good reason. Too, your motor most likely will not come up to full rpms in gear when tied to the dock even if it's perfectly healthy. Be careful!

--Tom.

 

[Hermit Scott]

Why don't you go for a spin, set the auto pilot then go below and mess with the throttle?
I wouldn't want to be throttleing up the boat while I was below deck while the boat is aimed at a $300,000 yacht on the other side of the marina.

 

[Franklin]

Because this area is completely full of boats. We have about 10,000 boats that sit in the water year round. We must have another 5,000 power boats sitting in sheds next to the water. Then there are those to launch daily. Our sailing grounds are more congested then the Houston Ship Channel and almost as bad as the freeways and that's only with 3% using their boats. I'm so glad most never use their boats.

 

[Franklin]

According the the formula (62.72 * Shaft HP * (Prop dia / 12)) ** .67, seems it's only 160 lbs.

 

[Don12364]

Hull Force - 3 Ways

On Main Sail's excellent posting "Fixed vs. Freewheeling Prop" I posted a curve done by the Wolfson Unit in Southampton of the hull resistance of a modern fin keeled 32' LOA yacht as 130Kg @ 7 kt. I scaled this (cube law based on LWL) for a 376.
see:- http://forums.hunter.sailboatowners.com/attachment.php?attachmentid=22992&d=1240173326

Next I looked in Prof Marchaj's book "Sailing Theory & Practice" and noted the graph on page 253 showing the hull resistance as approx 480lb for an older style heavy long keel hull of 45' LOA but with a 32' LWL.

Both of the above lead me to approximate the hull resistance as 400lb.

Then I used the propcalc formula which I obtained many years ago from a UK propeller company which gave a bollard pull of 750lb, a version of this can be downloaded from 'Castle Marine's" website in Wales - but the latter, unfortunately, does not show bollard pull.

Sheez this is hard work!

BTW Frankiln I have not previously seen your formula. Where does it come from please:-

According the the formula (62.72 * Shaft HP * (Prop dia / 12)) ** .67, seems it's only 160 lbs.

.

Lastly for "tsmweb" I own a 376, sister ship to Franklin's, and so am very familiar with the motor and prop as supplied by Hunter.

 

[Franklin]

I found it using google. I don't think it's right though, because no way could I hold the boat still with the engine even at 1500 rpm.

 

[tsmwebb]

[Shows his work]

Well, I'm impressed. Particularly cool that you could get the bollard pull numbers.

For the record I guessed at the hull parameters and ran them through the Delft interpolater function and then applied some Kentucky windage...

--Tom.