Long - Skinny Power Boats

Folks,

Reading the various design books that I have I've always been puzzled by why displacement hulled power boats aren't longer and skinnier?

I know the standard answers - moorage costs and interior "apartment" space. Both reasons are plus factors for short and fat boats. These are the sorts that stay (mostly) in a marina and are doo-dads to show off to friends/clients/bosses/underlings etc.

So ignoring the costs of moorage and loss of apartment space and speaking only on a theoretical basis…

From my reading it seems that a longer hull would equal a greater speed ... waterline length = V/L ratio = faster.

Skinny boats would seem to be inherently more efficient that shorter beaminer hull shapes, i.e. the same speed for less fuel or greater speed for the same fuel used when compared to 'conventional' hulls.

IIRC the Prismatic Coefficient for displacement hulls is somewhere around .56-.58. Regardless of my memory, does this 'rule' apply to hulls with a L/B ratio above 5:1? If it does apply, how would one achieve that Cp with L/B's above 5?

Though long skinny hulls don't have to be light, it seems that they would inherently have less displacement and therefore be less expensive to build, based on cost per pound being equal for any given length. True? Or is there some other factor besides total displacement that has a greater effect on build cost?

A few of drawbacks that I can think of – can anyone point out others?

The hull might be difficult to control in larger waves – the bow might bury in the wave trough while the stern is high on the last wave crest, thus being more susceptable to a breech or other nasties.

Spanning two wave crests – the bow and stern are `inside' two succeeding crests while the midships area is relatively unsupported in the trough, potentially causing the hull to buckle.

Quartering seas – whether from ahead or astern - might cause quite a bit of hull twist and make the boat perform some sort of a weird dance. This might result in control issues and/or hull failure from excessive twisting.

Beam seas – depending on the underwater hull shape – rounded bilges, multi or single chines or a V bottom or a flat bottom – each will react to beam seas a bit differently, but regardless of hull shape will have a pronounced roll – with the variable being the roll period – i.e., the snappiness of the motion.

How far off are these 4 points? Valid or not? Are these (and other) factors glaring enough faults (if they truly are) to preclude one from designing and building a long & skinny, highly efficient displacement power boat?

BTW, I'm trying to understand this for 'personal' sized boats - 35'-50', not naval destroyers or the like.

I don't have the answers; I'm just trying to learn. Discussion would be appreciated.

Best,

Leo

 

Size, room, and stability are some big reasons. Every boat has it's general design that is best suited for it's intended use, although there are exceptions.

 

Lots of long skinny displacement boats were built for a long time in years past. The drawbacks you mention were probably factors in the reasons that design trends have evolved in modern times. If you want to go faster or have better seakeeping capabilities there are now plenty of options to add horsepower or choose from a variety of hull designs. If you want to be more "fuel efficient" there are options for that too. There are compromises to all designs and the popularity of each is market driven. Long skinny displacement boats still have a place in the market but it is a very small niche.

 

Quite simply, additional longitudinal volume isn't as usable as extra beam space is. True, a long skinny hull is easier to propel, but the boat has to be sold to folks wanting elbow room so this compromise has worked into the mix.

Production manufacture's are mostly doing this, custom work usually takes a more owner friendly avenue. By this I mean custom designs tend to be less of an attempt to please everyone (fat) and more an effort to please the client's requirements, which generally produces more efficient shapes (skinnyer)

The call for efficient shapes in power craft isn't as necessary as it once was. Power plants have become much more efficient and effective at getting a craft to speed. With this, designs have beam and weight hung on them, knowing the power train can easily propel the craft to the target speed range. Years past, these more effective shapes had to be used, because the power plants available needed all the help they could get. These older engine designs were heavy, much less fuel efficient and produced much less power per pound then engines currently in use.

With modern propulsion units, hull forms can be wider without as big a penalty needing paying in performance and you get the more easily used elbow room.

 

As my pseudo shows, I'm very fond of slim hulls.
Let's talk only on a theoretical basis about slim boats (let's forget room etc) and will stay in small boats.

Like always there are advantages and drawbacks:

Advantages:
- high prismatic coefficients are obtained very easily. The curves are soft, with fine water entries and smooth water exits. The system of waves created by a slim hull is small, so less energy is lost to make waves.
- Under hull speed limit -the true one which is calculated on the basis of displacement (Gerr's formula which has a good precision ) and of coefficient forms of the hull (ratio length/width, prismatic coefficient, concave or convexe bow etc) as the formulae used for warships- slim hulls are very efficient: rowing shells, kayaks, multihulls etc are every day applications.
- Almost no hump in the drag curve before starting planning. A lot of heavy planing hulls need actually more power to start planing than for cruising. It's the famous "get through the hole" where a lot of overweight planing hulls stay stucked.
- As consequence these hulls need less power to get the intented speed in low Froude's numbers.
- Slim hulls have very good average speeds. Top speed is interesting, but good average speed is more interesting as you can maintain the speed whatever the sea condition. A well designed slim hull is seaworthy in bad sea, and where a wide planing hull will be miserable, slamming so badly on the waves thus forced to reduce its speed to ridiculous levels, the slim hull will be able to maintain a good speed.

For further reading on theory;
http://www.greenval.com/jwinters.html
http://www.cyberiad.net/hydro.htm of Mr L. Lazauskas, who is member of this forum, will give you a very deep explanation about slim hulls.

Drawbacks:
- slim hull means little righting moment ie transversal stability. This problem may be solved by different means.
- small mechanical inertia. Slim hulls need very good building; it's not a true problem.
- Disadvantage of the easily driven hull, slim hulls have tendency to cut the waves and to be "wet". Higher forward freeboard is needed. If the bow is too full you'll have strong vertical accelerations.
- While starting to plane the stern has a tendency to bury in the sea.
- Efficient planing need a wide waterplane shape; the shape of a slim hull is not efficient for planing, but very good compromises can be found.

Slim hulls were common at the beginning of the 20th century. The engines had very limited power and it was the way to get the max speed. A 1905 boat 12.2m long, 1.22m wide at the waterline, weighting less than 2 metric tons was able to get 16 knots with 16.5HP (a torquey engine 16.5HP @ only 650 RPM...But it was very wet, and turns had to be taken with precaution.

Solutions of the drawbacks:

Monohulls; one good solution is to have a slim underwater shape getting wider over the water by a pair of strakes. Paragon applies this solution on the VSV PATROL boats
( http://www.vsvboats.com/ ) The strakes give buoyancy and dynamic lift while the boat starts planing, a better planing surface (like a pair of skis), and insures safe turns.

Multihulls:

Powercats are well known, the australian use them extensively: it's the simpler answer to the problem of lateral stability, and room.

It's not a small boat but I cant resist to give the link: the world's fastest car ferry, "Luciano Federico L" competes with the airlines on the 110 nautical mile Buenos Aires to Montevideo route, at the cruise speed of 57 Knots full loaded.
http://www.amd.com.au/vessels/amd1130.php.

Power trimarans are the concept in vogue in warships and cargos/ferries. These tris are more monohulls assisted by outriggers than true multihulls. Nigel Irens ( http://www.nigelirens.demon.co.uk/index2.htm )initiated the trend with Ilan Voyager (21.3m long, wood and glass): 28 knots with a single engine of 180 kW (about 250 HP). this tri made a record-breaking un-refuelled voyage around Britain (1568 nautical miles trip in 72 hours 21.5 knots average with 2000 litres (530 gallons) of fuel (2.97 nm/gallon, the best ratio speed/consumption).

The 40 ft trimaran "YANMAR ENDEAVOUR" goes at 12 knots with 54 HP, has made the record of travelling 4443 nm crossing the Pacific with an average speed of 8.93 knots (4.8 nn/gallon). http://www.morrellimelvin.com/page73.html

The tris are very efficient; a small day tri would not present special difficulties of conception. We are far of a 30 ft 100% fatty "planing" boat with 2 engines of 250 HP sucking 24 gallons/hour (1 nm/gallon !) for a low 27 knots when it can plane...I won't name the builder.

Answering to your specific concerns:

Boats cost mainly per pound. Very true with a light small slim hull as the engine is the major expense on a such boat.

Larger waves; true and false. Depends more on the equilibrium of volumes between the bow and the stern and the undershape. It's not an issue.

Spanning two wave crests, Quartering seas; False. On a small boat is not a concern, we have now the engineering and materials. Hulls are not more made with nailed pine planks.

Beam seas; True and false. More than the rolling rate (the hull is slim so it will react less than a wide beam), THE TRUE ISSUE IS LATERAL STABILITY. On a slim monohull the righting moment is small if it has not features like strakes (sponsons) above the water. The better stability without compromising the speed is obtained with the multihulls, that explains their rapid growth in the market.

I hope I gave a beginning of answer.
Best.

 

MY DREAM SHIP !!

The bigest problem I envision is the inability to afford dock space, as its charged by the foot.
Some East coast places charge $3.00 per ft per night!!!

For a true cruising boat this would not be a problem as all systems etc would be stand alone , either underway or moored , with no Shore power required.
Just fuel & water service at times.

Plaining is EZ on a box boat (with mucho power), and dock space cheaper , so thats why we see so many boats "almost" as wide as they are long , and 3 or 4 stories high ,with "oxygen tent"-(fly bridge) stuck on top . Cheaper to dock an apartment house , and more room to cottage.

The thin cruiser should be able to carry much higher speeds with less fuel use than the box boats , making voyaging more affordable .

Always easier to get a few crew for a 2 week juant than a months , and weather windows are more accurate the shorter the forecast.
Less time, less fuel, less stores are all apealing to me.

To do it "Right" plate cooling (no keel cooler drag) , and good stability underway with out either stabilizers , or flopper stoppers would be helpfull.

Mitsubishi is releasing a gyro stabilizer unit in Oct , but I am not sure it would be right sized for a very slim boat.


Building costs and time (for me ) would require the compromise of building in sheet material.

Either aluminum , or GRP pre covered foam core where the sheets are stretched over the bulkheads and GRP is simply taped over the seams.

The trimiran shape would make loads of sense only if the amas could fold (Matamona style big complex $$$) or were carried so far aft a std travel lift (16-18ftwidth )could haul the vessel.
Tris do great at anchor in rolly harbors , so might be worth the extra expense & hassle for cruising.

Anyone sugest a hull that can be made developed ?

With our desires only 2 crew for costal and 4 for ocean work would be desired , so a loa of about 65 with 8ft Bwl would hold all the volume we need.

Width on deck should be -12ft Best or 14MAX , so the boat could be easily (cheaply) moved on land.

All help appreciated,


FAST FRED

 

A gyro is useless unless you have computers and a dynamic stabilizing device. With the price of a such device you have the fuel for the next 15 years. There are simpler and more reliable solutions for stabilizing a small boat.

A very slim monohull is more difficult to design than a cat or tri which are naturally stable. The study of the placement of the strakes, the calculation of the dinamic lifts, roll frequency and so are a good piece of work. The problem is that the US Navy or the DERA are not paying the bills and mistakes...

Folding amas on a medium sized tri is not a technological challenge. On sail boats the outriggers have a lot of stresses, but on a power tri these stresses are small. Several simple solutions are possible; sliding tubes, hinges, and others, but the solution designed by Farrier is the best, and I believe that the patents have expired...

The best (and simpler) architectural way for room is to consider a platform over the main hull, so you have not to make (bad) compromises with the hull shape which is the key of a successfull tri. Elliptical sections as those drawn by Nigel Irens (who worked a lot on the problem, and he is a master in multihulls design) are the best.

Compounded plywood or cylinder mold would be the simplest ( do not means the easier...) method but may be tricky: it is eyeball building. An hybrid method with bottom in strip plank (with a bit of kevlar for impact resistance) or simply monolithic FRP (too heavy, rigidity issue if kept light) with topsides in compounded sandwich, honeycomb or simply plywood would be simple AND easy.

Nidacore is perfect for floors, roofs, walls etc... it's price is more than competitive compared to foams and it's very easy to use because of the polyester fiber screening. Phenolic impregnated paper honeycomb Tricell is another alternative.

On a 35 foot I would stick to marine plywood/epoxy/glass with maybe a botton in strip plank; the ratio price/weight/strength/easiness is hard to beat. Aluminium is too heavy and complicated until 70-80 feet, but a Strongall (thick plates) structure is worth to be studied even on a 50 feet. http://www.reducostall.com/HTML/pagetwo.htm


Ilan Voyager is "cheaply" built in strip plank/glass and, after years of hard work in the rough seas of Cabo Verde (like a permanent tropical storm in Florida: 30 knots trade winds, 10 to 12 feet waves...) the boat is perfectly sound.

Thanks to Yipster for pointing out the "writing lacks". It's corrected now.

 

witch one or what is a "a Strongall structure"?
as said, tri's are not the easyest design and whats the price pic?
can i start saving leaving the boat in its slip?
no, seriously: tis is good stuff !

witch one or what is a "a Strongall structure"? ok found the link above now...

 

The Strongall sysem looks promising but the site doesnt give lots of info.

Is it similar to cold moulding , except in one layer and welded?

Had my first tri Headly Nichol Voyager 45 ft built in Belize British Honduras back in the 60's.

A great deal as 6000 man hours only cost $4,800.US

I have worked with Airex for a number of hulls , but the price is so insane (about $18 a sq ft in 3/4) it must be rulled out , however fine the hulls come out.

In playing on the computer the power requirements dont seem to grow out of range with simpler construction.
Aluminum or cored GRP is still first choice for resale, as in the US burried wood , or glass covered ply has virtually no resale value , no matter how finely done.

Anywhere in the world the world the Strongall system is used with low labor prices.

I looked into Poland for the std style built aluminum hull , but the legal system is not up to standards to not get shafted.

Sugestions anyone?

FAST FRED

 

I suggest that you look at the thread on the Ironheart design by David Gerr http://www.boatdesign.net/forums/showthread.php?t=1976&highlight=ironheart

At 66 feet long and 11 foot beam, it has a L/B of 6.6, cruises at 10 to 12 knots on 180 HP and has a 5,000 NMi range.

If you’re serious about making it transportable, then height will be a bigger limitation than beam. You can get an over height permit and go up to about 16' height, but then you need to carefully plan your route and your liable if you hit any bridges. I prefer to keep to the legal height of 13' 6". Actually about half the states have a 13'6" legal height and the rest have 14' (actually one state allows 14'6") but I digress.

Regards
Mike Schooley

 

Refining Long - Skinny Power Boats

Wow – Go sailing for the weekend and look what appears – lots of excellent posts! Thanks to everyone. Some comments from the various posts.

Propnut wrote: “there are now plenty of options to add horsepower” and “"fuel efficient" there are options for that too”.

Let’s try to define ‘efficient’ in this context. First, this definition is not about maximum accommodation volume, so we don’t design wide or tall – those options are out of bounds. Efficient here is the highest average cruising speed using the least amount of fuel. In my mind higher horsepower to go faster is not the way to achieve this goal. A long slippery hull that needs the minimum amount of horsepower to propel is at a V/L ~1.1-1.15 probably will give the highest average speed at the lowest fuel burn. Anyone disagree?

Par wrote: “True, a long skinny hull is easier to propel, but the boat has to be sold to folks wanting elbow room so this compromise has worked into the mix.”

Very true if it is a commercial product. This concept isn’t for sitting in a marina. This would be a one-off boat that harkens back to Beebe’s original Passagemaker concept. This concept is for folks that want to go places under power in a (relatively) quick and comfortable manner while maintaining a high average speed with low fuel burn. Beebe’s concept is ~50 years old and what advances have been made that will enable us to improve on Capt. Beebe’s ideas – if we are able to?

Ilan Voyager makes several excellent points and provides links to much information that is way beyond my feeble brain capacity to understand the mathematics of.

But I can’t help but wonder if the same principals that are being applied to canoes are also applicable for boats in the 10-15m range? As general theories I’d take a SWAG and say yes, but the caveat is that we’re talking about human power intermittently applied in the fractional horsepower range versus mechanical horsepower continuously applied in the 20-50 horsepower range. Are the apples still apples or have they transmogrified into some weird sort of orange?

Fast Fred wrote: “all systems etc would be stand alone , either underway or moored, with no Shore power required. Just fuel & water service at times.” And [snipped] “making voyaging more affordable.”

Per-zactly FF!

But then FF goes on: “"Right" plate cooling (no keel cooler drag)”

Why a liquid cooled engine at all? If we still want diesel for all its virtues, why not a Duetz Air/Oil cooled industrial engine? See http://deutzusa.com/Products/2011.html I’ll leave it to everyone to make up their mind if this ubiquitous industrial engine with its long service intervals and life makes sense for your particular concept.

If your ‘dream boat’ isn’t to be used for ocean crossings, (or even if it is) then why not consider a suitable 4 stroke outboard engine? Although somewhat new on the scene, they seem to be proving themselves (mostly) reliable.

Either alternative would obviate through hulls and their potential for flooding the boat.

Finally, Portager wrote about Dave Gerr’s Ironheart. I’d seen that design some time ago and even exchanged communication with Mr. Gerr (I heard a rumor that he died recently?) about the design. As of 2002 no one had yet built Ironheart.

Ironheart is a great example of the concept, but for me Ironheart is 6-9m too long and 10-12 tonnes too heavy. But the concept is excellent.

Finally, to echo FF, and dovetail in with the Portager concept, albeit with about 800-900mm less beam, what details would one design into the concept to make it suitable for:

The Inside Passage from Puget Sound to Skagway and back?
Trucking to New England for a summer’s cruise to Newfoundland and back?
Crossing the Gulf Stream to spend winter in the Bahamas?

Fast Fred mentioned sheet goods – and Ilan Voyager mentioned ply/epoxy. So instead of going with Ironheart in steel or aluminum – way too expensive – let’s see what can be suggested when we stick to simple hull forms and ply/epoxy materials so that we can home-build this creation. Maybe something like this from Reuel Parker, http://www.parker-marine.com/50dorypage.htm only 3m shorter and 1m less beam.

Again,

Thanks to everyone for responding to this thread.

Best,

Leo

 

You should probably take a look at the NPL series of hulls.

 

I searched RINA's archives and publications and came up with a fat zero.

Can you provide a link or some other information that will lead to the discovery of where one can obtain this information?

Thanks

 

Three reasons; reliability, fuel efficiency and hot water. Industrial engines may be reliable in industrial conditions, but they have proven to be less reliability in marine conditions where they are constantly sucking salt laden air through their insides. To accommodate wide temperature extremes with a less efficient cooling mechanism, air cooled engines need to make certain compromises, such as lower compression ratios to prevent premature combustion due to hot spots. These compromises equate to lower efficiency which is the bane of long range power boats. Finally liquid cooled engines provide a free source of hot water so you can take a warm shower once in awhile.

Plate cooling means the waste heat is conducted through the hull plate and no through hulls are required for cooling. Plate cooling works best on thermally conductive metal boats. Keel coolers are usually used on less thermally conductive boats such as fiberglass and wood/epoxy. With a keel cooler the coolant is passed through the hull to a heat exchanger on the outside of the hull and then back inside. It does require a through hull but the coolant loop is isolated from the sea water, so it would require two simultaneous leaks allow sea water in.

First diesel is intrinsically safer than gasoline. Diesel has a higher vapor pressure so it will not evaporate and create combustible fumes like gasoline will.

Second, the difference between a high reliability gasoline engine and a high reliability diesel engine mean time between failures is a factor of 5 or more (and the jury is still out on four stroke outboards). If you look at boats designed for offshore operation you will see that most gasoline engine boats have two engines and most diesel boats have a single engine or a main plus an auxiliary. This is because diesel boats only need one engine to make reliable passages and a single engine boat is more efficient than a twin. If your not sold on the single engine approach then consider a get home sail, or a wing engine/generator with a self feathering prop, or …

Third, outboards do not have the ability to heat water, so I repeat my previous statement about warm showers.

That depends on you and your significant other if applicable. Everyone has their standard of adequate comfort. In my case I’d definitely require a warm shower (there I go again), a real pilothouse, standing headroom throughout, an aft owners stateroom and air conditioning for those warm Bahamas nights. For a more detailed list feel free to look at Portager’s requirements http://www.portager.info/Portager_reqmts.htm and start crossing out things you don’t need.

I wouldn’t be too quick to jump to the conclusion that ply/epoxy is the best or cheapest option for home building. In the size range that you are talking about, aluminum would also be a good candidate (steel would be too heavy). Aluminum is higher cost per pound than ply/epoxy but since it has a higher strength to weight ratio it is not higher cost per boat and it will yield a lighter weight hull. Aluminum is harder to weld than steel, but it is as easy to work as ply wood and can be shaped with wood working tools. It also does not require any toxic chemicals or produce noxious fumes. Aluminum will also be easier to maintain and will retain a higher resale value than wood/epoxy. Finally metal boats are intrinsically safer in a lightning storm.

I would shy away from the 50’ Dory. Flat bottom boats tend to pound in rough seas. If your really interested in ply/epoxy construction then maybe you should look at Devlin Boats http://www.devlinboat.com/dcfp.htm . You should also consider the excellent designs of Tad Roberts http://www.tadroberts.ca/ .

Regards;
Mike Schooley

 

That's an interesting thread!!!!

I'm truly very happy that this thread is becoming a true exchange of opinions.
I'll try to give mines in some order;

Hydro:

- Hydrodynamics laws applies on all body going on the surface or inside the water like gravity law applies on any object. Corrections are made for scale of size and/or speed. An example illustrating the "universality" of hydro laws: the excellent Michlet software applies as well on canoes as destroyers or submarines.

The hydro links were given in purpose to show the first principles, and unhappily for the most of us, poor mortals, good design needs maths.

- NPL series from CD Barry; very good idea. If I remember well the NPL forms can be gotten roughly from sheet (metal or ply) and work very well in semi displacement. I'll check it and I'll try to find a link with pics. Let me the time to make a search on internet.

Another classic possibility is round bottom with chine (and maybe a small strake)

- sharpie forms (flat bottoms). I became allergic to flat bottoms while spending 6 months as lieutenant on an old mine sweeper. Flat bottoms make your life miserable when the seawater surface takes another shape than flat as a mirror. To have the pounding of planing boat without the speed is a fustrating feeling. The roll may be hard and "dry", but something will be always wet: the inside bottom. With only one bucket of water you have water everywhere inside...
This shape is not efficient for a semi displacement boat.

- Last in hydro but not the least: the ratio width/length. To exploit fully the advantages of the slim hulls you have to go to a ratio superior to 6. It's very hard to find the optimum ratio, which depends of these too numerous factors that make boat design a kind of art and not a tidy mathematical process.
But generally it lies somewhere between 8 and 10.

That explains why I prefer a multihull configuration with a preference for trimaran (one engine only) for a slim boat. A small monohull with a ratio of 8/1 would be dangerous.

Boat design has progressed a lot since Cpt. Beebe times (that does not retire any merit to the genius of Beebe). Simply look a the 2.97 nm/gallon at 21.5 knots and the 28 knots top speed with 250 HP of Ilan Voyager or the 4.8 nm/gallon at 8.93 knots and the top speed of 12 knots with 54 HP of Yanmar Endeavour... Nowadays a passagemaker is not boxing in the same category.

The following link

http://www.users.globalnet.co.uk/~fsinc/yachts/spreads/fred.htm

gives a hull resistance calculator (written in java) whose results are to be taken with great caution as these resistances are calculated for a very peculiar hull shape. For engineering I would use another soft, and I do so ... That does not retire any merit to this java soft, which is a nice piece of programming.

This calculator has a great educational advantage; for the same hull mathematical shape, you can play with the variables, and see the obtained resistances and the shape of the curves.

So keeping the same lenght, displacement, and prismatic coefficients, you'll see the influence of the width on the total resistance and the famous hump of hull speed limit. Very instructive... After you can play with the other variables.

Construction:

- Beyond engineering considerations, personal tastes and affinity guide in part the choice of the material. Personally I do love work with wood, plywood and epoxy as I begin as aprentice in a shipyard working these materials.
I do hate the smell of polyester so I'll never use it. I do like aluminium but I think that all the work and cost of the insulation annihilates any advantage in small boats; a metal hull without a thick insulation under the tropics, no thanks.

Maintenance on composite wooden boats is not an issue if you use enough epox and fibers. The boats built in strip plank and plywood with biaxial and UD age very well.

Lightning is not a concern if you take the precautions needed whatever the hull material.

- Personal tastes guide the choice of accommodations and amenities. I'm rather on the spartan side, and I have tendency to see things like air conditionning as potential nuisances. As I'm definitely against cold and wind after a few years spent on warships in North Atlantic I prefer a tent and hamacs over the deck while at mooring close to coco trees. Personal taste again.

- Size: by experience, and from the experiences of many others I think that around the 40-50 feet is the max size you man manage with a good probability of success for a homebuilt boat. However my thought is not written in stone by God's finger.
After the project becomes very big. Or the size of your wallet permits other building options.

Engine:

- 4 Stroke outboard is a good option in recreationnal use near the coast.

- Industrial air cooled engines may be used as it's done on small fishing boats
but the first drawback I see is noise. Air cooled engines (oil cooled is a variant) need big blowers, big openings and big exits for the big air flux...it's very difficult to get a whispering engine when it's air cooled. Sound dampening becomes almost impossible.

The dry escape may be a security issue in a tight engine compartment. Salt coming with the cooling air will cause corrosion. So a fresh water cooled engine is preferable with the simpler cooling system: heat exchanger.

Against common belief, surface propellers may be a good option for a semi displacement boat. Efficiency, simplicity, little draught, "clean" bottom, easiness of maintenance of bearings and rudder are some of the advantages of a simple fixed surface propeller in the kind of the Levi drive.