figureing HP

Is this a good resource to go by?

http://files.dnr.state.mn.us/education_safety/safety/boatwater/backyardboatbuilders.pdf

is there a reason a designer would derate the HP on a given boat what the scantlings are maybe ?

 

It would be more helpful if you made your question(s) clear. As it is we can only guess as to what you're asking.

There are many reasons a designer might use a lower HP recommendation that what may be permissible by the USCG.

Is there something specific you are trying to do, such as repower your boat? If so, the year, make and model will be a good start. As would a target speed, the general conditions you use the boat in, it's general use as a vessel, etc. There are several variables that go into HP recommendations, so a simple chart isn't possible, but there are some very basic guidelines that can be employed.

 

The pamphlet you referenced is for monohull (one hull) outboard powered boats under 20 feet in length. Is that the type of boat you are asking about? It doesn't apply to larger boats, catamarans, pontoon boats and inboards, inflatables, canoes, kayaks , etc.

If it's a different type of boat then Safety Standards For Back Yard Boat Builders is not a good reference.

Why do manufactures down rate? First the USCG formulas for rating o/b boats are general rules for a wide range of hull types. Here's a short list of reasons for downrating.

The boat doesn't handle well with the maximum rated HP
The boat can hold more people with a smaller lighter engine
There simply isn't enough space for flotation for a heavier engine.
Liability. The manufacturer wants to limit their liability.
Speed is not the main issue. Maybe the boat is designed for a use that doesn't need speed.

There are more. But the point is, when designing a boat there are many compromises that need to be made, HP is just one of the variables.

 

I'm sorry for the delayed response we have been without power the last coupla days we have been cleaning up as well thanks for the responses guys , I am not really re powering any particular boat I was using the resource listed to look at what power could be used on any given boat the boats I was looking at were a couple of Bolger boats one was Diablo one was seahawk in the text Bolger said that Diablo handled 25 Hp easily. I am not positive I am doing the math right on figuring HP but it looked like the CG was saying diablo was capable of handling 35-40 HP but the scantlings on a lot of Bolger boats seem light to me but they turn out to be light weight boats. sorry for not providing details in my original question again thanks for the insight guys

 

"Scantlings seem light to me". In what reference can you ascertain this logic? In other words, without back engineering the loads imposed, "taking a guess" isn't going to be remotely close to a structural assessment.

As a rule the USCG HP suggestions are just that, suggestions and many designers find the amount of power the USCG says you can place on a boat way out of line with reality. I have a simple flat bottom design, that according to the USCG can handle a 70 HP outboard. It runs great with a 20, scoots damn fast with a 30 and gets down right scary with a 40. A 70 places it into holy grail territory, where you want Saint Christopher himself sitting in your lap with you, at the speeds this recommendation will generate.

This happens fairly frequently, because they use a generic formula, to determine the max HP figures. Some boats don't fit well into this "formula" so you'll see less then the USCG slated HP figures spec'd for the design.

The Diablo is a fine little boat and yes, it's light, which is the whole point. This lightness permits a smaller outboard to produce the same preformance, as a bigger one on a heavier boat. If you increase the scantlings on Diablo, you'll probably kill it's preformance envelope and likely need the usual amount of HP for a power skiff in this general size. In this case, what's the point? In small craft weight is a preformance robber and Bolger was very skilled, in creating scantlings suited for the tasks, without burdening the vessel with unnecessary weight. This has the added benefit of making the materials list less costly and the build easier too.

 

mostly just comparing his designs to the flat bottoms I grew up in that had 2 X 4 for chine logs and framing to match

I just have a curious mind not really wanting to do any of what you are describing I was just wondering why there would be a discrepancy between what Bolger suggested and the CG's math I really appreciate you taking the time to further the little bit I know about boats thanks

 

The discrepancy between the USCG HP recommendations and many designs (again) is they use a blanket approach. The same "math" is used on fat short boats, long skinny boats, light weights, heavy beasts, they're all treated the same and the result is often unacceptable, unless the design fits neatly into the little box they've designed the math around.

When it comes to scantlings, it's very difficult to compare builds of different types and even those of the same type. For example you can use thin planking with closely spaced frames or you can use thick planking and widely spaced frames, both on the exact same hull form. It doesn't mean one is more "dainty" then the other, it's just a different engineering approach. Once you cross over this line, you can easily encounter the exact same hull, but with not a single frame and a monocoque hull shell. This doesn't mean it's weak, it's just a different engineering approach.

As a general rule, boats of our youth can be built much lighter then they once where, with modern techniques and in some cases newer materials. I grew up with thick, file planked bottoms, 2x4 chine logs and massive deadwood assemblies too. Now, I'm designing boats with no chine logs, foam filled, lightweight appendages that replace the massive deadwood assemblies and have long ago tossed out the need for file planking.

In 1900, a ten HP engine could be 500 pounds, now we can have 500 HP with the same weight. Same reason as you see in the scantlings of boats, we've learned and have gotten better at it.