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Boat HP rating

 
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jrslost@aol.com



Joined: 02 Mar 2023
Posts: 4
City/Region: Pensacola
State or Province: FL
C-Dory Year: 2005
C-Dory Model: 25 Cruiser
PostPosted: Thu Mar 02, 2023 8:33 pm    Post subject: Boat HP rating Reply with quote

horsepower rating for a boat. For example, if my boat rating is 150 hp max, is approximately 500 pounds on the transom. if I put dual 75 hp motors for total of 150 hp at 400 pounds each is equal to 800 pounds on the transom. Logic is telling me the rating for the boat would be transom weight, not horsepower. Prove me wrong.
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JamesTXSD



Joined: 01 Mar 2005
Posts: 7430
City/Region: Tropical Tip of Texas
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C-Dory Year: 2007
C-Dory Model: 25 Cruiser
Vessel Name: "Wild Blue" (sold 9/14)
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PostPosted: Thu Mar 02, 2023 8:45 pm    Post subject: Reply with quote

There are CD-25s that have 200hp or twin 90s, that came from the factory with that setup. Proof enough?
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jkidd



Joined: 23 Oct 2006
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Vessel Name: Voyager
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PostPosted: Thu Mar 02, 2023 9:19 pm    Post subject: Reply with quote

Factory told me it was weight not HP that mattered.
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ssobol



Joined: 27 Oct 2012
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City/Region: SW Michigan
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C-Dory Model: 22 Cruiser
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PostPosted: Fri Mar 03, 2023 12:46 pm    Post subject: Reply with quote

Besides the weight, wouldn't higher HP engines put more stress on the transom. Higher thrust=higher torque moment trying to pull the transom off.
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jkidd



Joined: 23 Oct 2006
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PostPosted: Fri Mar 03, 2023 3:26 pm    Post subject: Reply with quote

ssobol wrote:
Besides the weight, wouldn't higher HP engines put more stress on the transom. Higher thrust=higher torque moment trying to pull the transom off.



Or does it just move the boat so it can't put any more stress on the transom?
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ssobol



Joined: 27 Oct 2012
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PostPosted: Fri Mar 03, 2023 4:18 pm    Post subject: Reply with quote

jkidd wrote:
ssobol wrote:
Besides the weight, wouldn't higher HP engines put more stress on the transom. Higher thrust=higher torque moment trying to pull the transom off.



Or does it just move the boat so it can't put any more stress on the transom?


On an outboard motor the thrust line is below the bottom of the boat (where the prop is). When the motor is producing thrust this puts a torque on the transom that wants to peal it off from the top. To not have a torque moment, the thrust line (prop) would have to be at the same level as the motor mount to the transom.
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jkidd



Joined: 23 Oct 2006
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PostPosted: Fri Mar 03, 2023 6:24 pm    Post subject: Reply with quote

ssobol wrote:
jkidd wrote:
ssobol wrote:
Besides the weight, wouldn't higher HP engines put more stress on the transom. Higher thrust=higher torque moment trying to pull the transom off.



Or does it just move the boat so it can't put any more stress on the transom?


On an outboard motor the thrust line is below the bottom of the boat (where the prop is). When the motor is producing thrust this puts a torque on the transom that wants to peal it off from the top. To not have a torque moment, the thrust line (prop) would have to be at the same level as the motor mount to the transom.



When my boat was built in 2007 the yellow tag said max was 100hp. I ask the factory to mount a Suzuki DF-115 That is when they told me it was weight not HP. In 2021 I had Sportcraft mount a DF-140 on my boat and they called the factory to get approval the factory said it was ok and Sportcraft mounted a motor that was 20 lbs lighter than my 115. I did it because the boat is heavy loaded or used at altitude. For the 22 and 25 the yellow tag is a recommendation not a requirement you can just peel it off and that is what the factory did. They never installed the tag. My insurance company just says what boat and what size motor and sends me a bill. Pretty sure if my policy says what the boat is and what the motor is and the insurance company took my money there is a contract.


While I agree that a larger motor is going to apply more torque to the transom I don't think it would be 100 percent because the boat moves forward. You know for every action there is an equal and opposite reaction. ( Newtons Law ) That being said I seldom run at WOT 20-22 MPH or 16-18 MPH and there is always a little extra in case I need it.
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T.R. Bauer



Joined: 17 Nov 2007
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PostPosted: Sun Mar 05, 2023 12:03 am    Post subject: Reply with quote

This gets discussed a lot...never seems to die actually. I honestly don't think about it much as the transoms are very sturdy and well engineered. If you were worried, you can always add in some knees to help support the weight and dynamic loads. I would think the issue and concern is weight more than anything. These boats are pretty light and weight off the back of the stern is probably the worst place you can have it. It will eventually impact handling - no idea what the point of no return is on that, but it's probably about 500-600 pounds like the factory said. I certainly wouldn't have 90s on the back of a CD 22. I suspect those sexy (and light) 70s Yamaha makes are the max - absolutely more power than you need.
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Hunkydory



Joined: 28 Mar 2005
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PostPosted: Mon Mar 06, 2023 1:48 pm    Post subject: Reply with quote

One of the many deciding factors, when I chose to go from the Honda twin fortys to the Honda twin 60s was C-Brat, Lew of CAT CLAWs decision to go from twin Honda 35s to a Honda 115, which the factory was fine with his doing & I think fine with adding a kicker to that. Lew is pleased with the Honda 115, but I noticed it weighed exactly the same as two Honda 60s at 478 lbs & with the twin 60s there was no need for the additional weight of 50 to a 100 lbs for a back up kicker.

The sea conditions, myself & others have experienced & shared, show a CD22 is very versatile in handling high weight variances. This inherent ability has been a good part of making some of our more extreme adventures safely possible, while going from absolute boating novices to very experienced, over the last 20 years.

Jay

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thataway



Joined: 02 Nov 2003
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PostPosted: Mon Mar 06, 2023 9:33 pm    Post subject: Reply with quote

Consider the design of the hill has a role in weight. (Broad beam aft& hard chine as examples. Also hull design factors in horsepower. Also construction of the transom.

The layup of the laminate, any stringers or knees tabbed into the transom. Will contribute to both weight & HP.

Transoms have failed with excessive HP.

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Bob Austin
Thataway
Thataway (Ex Seaweed) 2007 25 C Dory May 2018 to Oct. 2021
Thisaway 2006 22' CDory November 2011 to May 2018
Caracal 18 140 Suzuki 2007 to present
Thataway TomCat 255 150 Suzukis June 2006 thru August 2011
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