Semi-displacement hull, HP at displacement/hull speeds

If you take a design like Devlin's surf scoter 22 cruiser

http://store.devlinboat.com/surfscoter22cruiser.aspx





and you only want to cruise the boat at displacement/hull speeds of 6-6.5 knots so you don't have to purchase a larger engine and you are more interested in efficiency instead of speed....

How much HP would it require?

The fact that the boat is designed as a semi-displacement hull, does this have any affect on efficiency or performance when only running at displacement/hull speed?

 

Yes, Quite a lot. And since you have an OB you have quite a bit of weight aft and that insures you'll have more transom drag. And w a gasoline engine efficiency is much less at lower throttle settings. A full displacement hull your size w diesel power should burn about 1/3 GPH. And probably less than a gallon an hour if a gasoline engine is used. On the bright side you're very low displacement will be to your advantage. But at 6 knots you may burn more fuel than my 8 ton full displacement boat and it is'nt optimized for fuel efficiency.

 

Displacement speed is about 6 knots and under on a 22' like this. In calm conditions it will take about 1hp for every 500 pounds of displacement to make hull speed. This looks to be 10hp for the Surf Scooter. You may, or may not, want more horsepower when things get rough.
Underpowered boats have poor resale values. You could use a 10, or possibly 15hp engine and a future buyer (or you) could install a 90hp main and use the existing engine as a kicker. Any engine between 15 and ~90hp will just burn fuel and not make the boat plane efficiently.
I'd go with the 10hp engine as it's likely lighter than a 15hp and, assuming the small engine is retained, won't add excess unnecessary weight when the 90hp engine the boat was designed for is installed.

 

Just another consideration from me, to weigh agains other arguments:

A difference in weight between a 10 HP and 90 HP outboard, taking Mercury o/b engines as examples, can be up 130 kgs (290 lb): http://www.mercurymarine.com/compare/outboards/?compare=25,8
I've checked it for Yamaha models too, and the numbers are very similar.

If the aft part of the hull has been designed to carry a 90 HP engine, then you might as well end up with a much less submerged transom (stern-up trim) if you mount just 10 HP instead of the original 90 HP. The prismatic coefficient will also likely be lower for a stern-up trimmed hull like that.
Running with a smaller wet transom and lower displacement would then mean a smaller drag at low speeds than the (otherwise correct) analysis by Easy Rider implies - a drag perhaps even comparable to a double-ender hull (just guessing, in absence of more hull-shape-related data).

A more in-depth hydrostatic and trim analysis could give some useful numbers.

Cheers

 

Why don't you ask the designer/supplier of the plans?

 

Why don't you contact Sam Devlin and see what he knows about Surf Scoters with smaller outboards? There is an article on Devlin's website by someone with a Surf Scoter 22 which had an engine good for 12 knots. http://devlinboat.com/eric-hutchinson-surf-scoter.php

 

Looks like Ad Hoc and I had the same thought.

Anyone with an estimate of how much higher the drag than a similar sized boat without an immersed transom? Double the drag? 10% higher drag?

Devlin designed the Surf Scoters for outboard and I/O power so I expect the illustrations show the amount the transom will be immersed with the recommended engine.

Modern 4-stroke outboards, particularly the 40 HP and higher models with computer controlled multi-port fuel injection, have much better part throttle efficiency than older 2-stroke outboards.

 

I did contact Sam Devlin.

Said the boat would be fine at displacement speeds.

60hp High Thrust - 15knot top speed, 10knot cruise
25hp High Thrust - 8knot top speed, 6.5 knot cruise

Then I asked about using the Yamaha 9.9hp High Thrust for 6knot cruise and he said, "Most likely it would work fine..."

I posted here because I wanted to get some other opinions.

 

What if everyone disagreed with the advice given to you from Sam?..what would you do??

You need to be careful, and also fully understand your motivation for seeking "other opinions" and then what you would do, when faced with contradictions.

If you deviate in any way from the advice given by the designer of a boat, for a change that you are requesting, then only you are responsible. The designer shall absolve him/herself from any repercussions.

Hence, what is your motivation to seek other opinions when you have one from the designer, and what will you do about contradictions?

 

I assume Sam Devlin's numbers are based on experience. I doubt guesses here would be any better.

A 9.9 HP outboard at full throttle may use more fuel than a 20 HP or 25 HP outboard at the same boat speed, assuming the props are appropriate.

 

I'll pitch in with the obvious question... why not go for a design that's designed to run at displacement speeds? If you are going to build from scratch, then them most sensible thing would be to build to a design that is suited to your needs.
One of the most important aspects of yacht design is to identify exactly how you want to use your boat, what you want to do with it etc... It's called a Statement Of Requirements, or SOR. Spend some time getting this right, then set about finding a boat that meets your SOR. Anything else is likely to wind up in dissapointment....
That's not to criticise the boat in question in any way of course...

 

I agree 100%, but there aren't many true displacement hull designs to choose from that meet my requirements. 19-22' displacement hull, OB motor, VBerth that actually fits 2 six foot persons, pilothouse for two people with enough space to sit/some storage/tiny galley, building material/style (stitch&glue or plywood on frame, maybe plywood lapstrake or strip but I am not familiar with these styles of building), then to top it off I have to like the aesthetics.

There are some designs out there, but only a small handful.

Bateau HMD19, front runner right now, simple/straight forward, I personally think it looks good. Have plans, working on model and drawing out pilot house and Vberth full size, realizing how small this boat is. If I don't find anything better that fits my requirements, this is what I will build.

Bateau MM21, aesthetics.

Tad Roberts 19.5 pogy, I am getting more information about her, her looks are growing on me.

Tad Roberts TimberCoast 22, diesel.

Hartley Fisherman 18, smaller size, doesn't really do anything better than HMD19 IMHO.

Gartsides Jennifer, diesel

Paul Fishers Rufus, aesthetics.

Devlin Dipper 19, Dunlin 22 cruiser and Surf Scoter 22 cruiser, have study plans for all, really like the Dunlin and Surf Scoter but are semi displacement designed for high HP. Dipper 19 is similar to Bateau's HMD19 but is also a semi displacement hull.

Glen-l has a couple tugs, not really into tugs.

Am I missing any designs/plans?

 

So what would be the disadvantages of Surf Scoter at displacement speeds compared to a similar boat designed for displacement speeds only without the immersed transom? There would be an increase in fuel consumption but the best guess is it will be a fraction of a gallon per hour, and my guess is a relatively small fraction. Other disadvantages? I'm asking about Surf Scoter in particular, not any and all designs labeled "semi-displacement".

 

Does the V-berth in the HMD19 really fit two six-foot people? It's listed as 6' 6" long but converges to a point so not all the length is usuable. I'd at least mark the space on the floor and try laying in it with another person.

Have you looked at Karl Stambaugh's designs at Chesapeake Marine Designs? http://www.cmdboats.com/index.htm His Redwing 21 Pilothouse appears to meet your requirements. If you want something larger he has the Launch Cruiser 24 which as designed uses an inboard 9 HP Diesel but you might ask Karl about the feasibility and cost of converting it to outboard power.

 

I went though that exercise a year ago for an SOR very similar to sean27's SOR and found there isn't much out there in either stock designs or exisiting boats. I then considered "semi-displacement" boats and concluded the difference in fuel consumption for the Surf Scoter series and similar would be small and not significant for us.