BoaterEd Store      - Help Support This Forum - Join Today!      Hunting/Fishing Stuff
BoaterEd
Username:
Password:
Save Password


Register

Active Topics | Active Polls | Resources | Members | Online Users | Avatar Legend | Statistics
[ Active Members: 7 | Guests: 104 ]  [ Total: 111 ]  [ Newest Member: luna sea ]
 All Forums
 Forums
 Sailing and Sailboats
 Is there a calculation to determine horsepower?
 New Topic |   New Poll New Poll |   Reply to Topic | 
Author Previous Topic: Cat boat sailing Topic Next Topic: Americas Cup  

Blues Cat

RO# 30222

Posted - Oct 15 2010 :  12:39:14  Show Profile  Reply with Quote
Iím starting the research on re-powering my 1979 Cal 31. Just how much horsepower do I need to move a 10,000 pound boat against a current? The MD7A has 13 hp and it gets me around. With all of the ratios and calculations out there is there one for sizing engines?
Thanks, Jeff
1979 Cal 31 Hull #8
"Bonnie Lou"
Kemah, Texas

Homeport: Kemah, TX

Radioactive

RO# 3238



Posted - Oct 15 2010 :  15:42:03  Show Profile  Reply with Quote
There are a couple of ways to handle this question...

1) Call the builder and ask the guys who designed the vessel. They have all of the info required to make the calculation.

2) David Gerr's book "The Nature of Boats" has a full chapter ( Chap. 20 ) devoted to the question.

One issue you have is that the hull of your boat is almost certainly a "full displacement" type. Further, modern sailboats employ several "tricks" to increase the maximum speed you can get out of this type of hull. This means tat the "rule of thumb" calcs for hull speed and effective speed will likely produce large errors. In any event, you will not want to try to push the boat at a speed in excess of hull speed ( in the real world, you will not succeed ).

--

So what is the purpose of the re-power? If you are replacing the current engine, it is likely that it is well-sized. Adding power to an un-powered hull will require the steps mentioned above.


Bill

"No matter where you go, there you are." -- Buckaroo Bonzai
"There is nothing, absolutely nothing, half so much worth doing as simply messing about in boats." - Kenneth Grahame

Homeport: MS Gulf Coast Go to Top of Page

pdecat

RO# 842



Posted - Oct 16 2010 :  07:58:43  Show Profile  Reply with Quote

the current is irrelevant. You need enough power to move the boat through the water at its maximum speed. You will never exceed hull speed.

If your current power is capable of pushing the boat to the point where there is a bow and stern wave with a trough in the middle that is as fast as it will ever go. If there is a counter current you many not move or even go backwards.
A little extra power for waves and wind would be nice IMO. More importantly IMO for sailboats is sufficient power to re accelerate after a wave slows or stops the boat and also to overcome wind age.


Bruce



Edited by - pdecat on Oct 16 2010 21:35:03

Homeport: Gulf Coast FL Go to Top of Page

Radioactive

RO# 3238



Posted - Oct 16 2010 :  18:34:13  Show Profile  Reply with Quote
I found another, perhaps more useful reference ( "Understanding Boat Design" by Ted Brewer ):

For sailboats, he states that "3-4 bhp per ton is adequate, 5 bhp per ton is ample and 6 bhp per ton is often too much."

In which case 50 bhp seems to be "about right".

According to his rule of thumb... ( Frankly, this is outside of my personal experience. )


Bill

"No matter where you go, there you are." -- Buckaroo Bonzai
"There is nothing, absolutely nothing, half so much worth doing as simply messing about in boats." - Kenneth Grahame

Homeport: MS Gulf Coast Go to Top of Page

saltysam

RO# 26



Posted - Oct 16 2010 :  19:38:03  Show Profile  Reply with Quote
Many sailboats of that era had small Perkins or Westerbeak diesels--4-107 or 4-108 models. They were usually rated at 25 or 30 HP. Original equipment in many cases. Did the job.


Cheers!
Bill

Edited by - saltysam on Oct 16 2010 19:39:20

Homeport: New River, Ft Lauderdale Go to Top of Page

stmbtwle

RO# 7934

Posted - Oct 16 2010 :  20:00:03  Show Profile  Reply with Quote
All of the above. How fast is the current and how fast do you hope to go against it??? If it's a 5 kt current and you want to make 5 kts, that's 10 kts through the water; for a 30 footer you're going to need a LOT of horsepower (probably more than you can put in the boat). If it's a one knot current and you only need to make one knot, you won't need much.

First figure out how fast you want to go THROUGH THE WATER...


Willie: Look Ma no paddle!

Homeport: Tampa Bay, FL Go to Top of Page

stmbtwle

RO# 7934

Posted - Oct 17 2010 :  07:18:09  Show Profile  Reply with Quote
Easiest way is ask your neighbors with similar sized boats what they have, and whether they're happy with it. Too much power will be a waste.

Willie: Look Ma no paddle!

Homeport: Tampa Bay, FL Go to Top of Page

Sandy

RO# 1159

Posted - Oct 20 2010 :  14:53:20  Show Profile  Reply with Quote
quote:
Originally posted by Radioactive

I found another, perhaps more useful reference ( "Understanding Boat Design" by Ted Brewer ):

For sailboats, he states that "3-4 bhp per ton is adequate, 5 bhp per ton is ample and 6 bhp per ton is often too much."

In which case 50 bhp seems to be "about right".

According to his rule of thumb... ( Frankly, this is outside of my personal experience. )



Radio- If it's a 5 ton boat, don't you mean ~~25 bhp ?


Sandy

Homeport: The Vineyard Go to Top of Page

Radioactive

RO# 3238



Posted - Oct 20 2010 :  15:18:31  Show Profile  Reply with Quote
--> Sandy

Hmmm. Too much coffee. You are correct. 25 +/- 5 bhp.


Bill

"No matter where you go, there you are." -- Buckaroo Bonzai
"There is nothing, absolutely nothing, half so much worth doing as simply messing about in boats." - Kenneth Grahame

Homeport: MS Gulf Coast Go to Top of Page

Blues Cat

RO# 30222

Posted - Feb 10 2011 :  07:29:17  Show Profile  Reply with Quote
Judging from the numbers I've found, I think I'll just nurse the old diesel along for a little while longer. Hey, the darn thing still works! I'll adjust the valves and wipe it off with some degreaser and call it good for now.

Thanks, Jeff
1979 Cal 31 Hull #8
"Bonnie Lou"
Kemah, Texas

Homeport: Kemah, TX Go to Top of Page

Vic Willman

RO# 3655

Posted - Mar 02 2011 :  09:55:43  Show Profile  Reply with Quote
Our 1979 Pearson 36 Pilothouse (same hull as a 365 but bigger cabin area and HEAVIER) came from the factory with a 40 HP Westerbeke diesel, which did the job nicely. So 25 HP should do it for you...



-- The Head Master --

Homeport: Millville, NJ Go to Top of Page

Billylll

RO# 24494

Posted - Aug 08 2012 :  21:27:31  Show Profile  Reply with Quote
Another important often overlooked issue with sailboats used as motorsailors is adding the horsepower for heavy duty charging (200+ amps), refrigeration, a watermaker and hydrolics. If your going to really update the boat and change the original motor out I would consider the items in my list that you would like improvement or want to add to the vessel.
Bill


Bill Lentz
Little Egg, N.J.

Homeport: Tuckerton, N.J. Go to Top of Page
  Previous Topic: Cat boat sailing Topic Next Topic: Americas Cup  
 New Topic |   New Poll New Poll |   Reply to Topic | 
Jump To:
BoaterEd © BoaterEd Go To Top Of Page
This page took 0.47 seconds to load
Forum Guidelines and Privacy Notice

    

Boatered.com