Inboard Engine on Inflatable

[mistyreiny]

I do not have this boat yet, but I have rhis one in mind for the reasons posted below. The engine, I have.

L70V Engine: 4.8 KW Yanmar diesel.
6.4 Hp
0.4gph= 1 gal per 2.5hrs @ w.o.t. (3600 rpm) 13.5NM= 10ft lbs
80lbs
0.32L


Mercury Achilles 7'
Hypalon inflatable

Supposedly I need a stern tube, wtf is that?
I'm just thinking of w.o.t. I get 2.5 hrs per gal.
Sounds like a seaworthy vessel to me! More like a bay cruising, sandbar, clam hunting and crab pot picking boat.
Any advice of acquiring a stern tube so my diesel willy motor can drive a prop and cruise around at 10-15mph?

 

[kapnD]

Unless the boat has a hard bottom, inboard power is a very unlikely candidate.
Even then, at a mere 7’ with a motor in the middle, there will be little to no room left for things like the captain and crew, and their belongings.
The stern tube carries the driveshaft to the wet side.

 

[mistyreiny]

Thank you for thinking critically. My rebuttal is I’m not expecting to move a crowd with the dinghy. Me and my fiancé are all I expect. Use the dinghy to check some crab pots in the Indian River Bay.
Therefore this renders your solid point of little space a moot point since I had not said my intended purpose. By no means is it a party boat, lol.
Hard bottom: solid point I had overlooked. I had assumed it would have a solid bottom, which would make rolling it up very difficult. Now my attention is directed to this problem, easy fix.
Give it a removable hard bottom. I have great skills and capacity with wood working, though this time would be manufactured marine plywood. Speculating here: 3-4 separate pieces, 1 each for bow, transom, 2 in middle. Either a middle or transom piece would have 4 ‘bolts’ sticking vertically. ‘Bolts’= quick release latches akin to your bicycle seat height adjustment.
The floorboards fit in with tongue n groove. Then there are two skinny lengths running boats length, pressed up under boats’ side tubes to add rigidity using ‘bolts’.
All in all, this produces a removable yet rigid floor with engine mounts. Transom has non removable, non adjustable stern tube.
Space will be highly limited for cargo, but what do you expect from a 7’ inflatable boat with an inboard motor?

 

[mistyreiny]

My back porch is azek manufactured wood. I could easily imagine making the boat floor out of it. Lasted great over 15 years in many onshore weather conditions. Using identical wood? No. Similar. I can only imagine Azek makes a comparable marine 5.5” x 6’ plank.
Imagine making a portable seaworthy boat where the heaviest part is 100lb (boat hull), and next heaviest is 80lb engine. Granted the term seaworthy is highly judgemental.

 

[messabout]

Your engine installation is going to weigh a lot more than 80 pounds. You'll need a rudder, a cooling system, exhaust system, fuel tanks, and such. All that weight plus the hull, plus the hard bottom, plus two passengers and gear..........and you will have a total weight of more than a seven foot rubber dinghy is likely to put up with. A stubby boat that is as overloaded as this one will is going to need more horsepower than you engine can produce, in order to get 10 to 15 knots. Think 6 knots and be sure to wear your wet suits.

 

[bajansailor]

Excerllent responses above!
Your best bet, if you are determined to find a use for this little diesel engine, might be to try to find a 'small' (but at least 14' long) fibreglass motor boat hull that is in reasonable condition and is available very cheap (or ideally free).
You would then still have to buy a propeller, prop shaft, coupling, stern tube, P bracket (to support the shaft), a through hull fitting for the cooling water and perhaps build a transom mounted rudder.
Perhaps even a suitable reduction gearbox for the engine (you do not say what it currently has).
The cost of all of this will be significant.
Alternatively, just use this engine for powering an emergency generator at home?
Or sell it, and buy an outboard motor instead for the dinghy that you fancy?

 

[skaraborgcraft]

A suzuki 2.5hp outboard on an inflatable apparently holds a record of 63mpg. Try not to overthink it.

 

[mistyreiny]

Engine Water cooling system. N/A. It’s air cooled. The engine has no reduction gearbox. However, it makes 13.5 NM (10ftlb) torque @3000 rpm. As for 3xhaust system. You saying I shouldn’t let the engine 3xhaust feed straight to the atmosphere as it already does?

rudder: I’m just plannin* on this. See red boat pic below. That rudder can’t weigh more than 20lbs. Fine, let’s call it 30lbs to be safe.
I do recognize the pictured rudder, right up against the stern, won’t work with having a boat prop off the stern.
I’ll just extend the rudder out 12” to give it a 12” gap between stern and rudder.

 

[skaraborgcraft]

Atkin & Co. - Bagaduce

If you are handy with wood, just build a simple skiff.

 

[mistyreiny]

I claim to be handy with wood, yes. But building a watertight skiff is next level engineering for me. I’m not saying I couldn’t build a watertight skiff but for the learning curve and time requirement, I’d rather not.
What I (meaning my father) have, is a 25 y.o. Tablesaw, miter saw, edging machine, drill press, my brother has a 2’W sanding belt, and all the equipment and knowledge to run his business Live Edge Designs. Oh, did I forget my father bought my brother a 20’-25’ ‘portable’ sawmill.
There is, rather was, a 2” trailer hitch on the front of that mill. First thing my father did was pour a concrete pad beside his workshop, park the mill on it, level it, took a chopsaw and removed the 2” trailer hitch hookup, built some walls and a roof around it. The electric company renting commercial space next door (from him) was cutting up their old telephone poles into 4’ sections and throwing them away.

I think Skaraborgcraft implies a punch a hole through the floor of my inflatable, mount the engine at a 45 degree angle, or flat with a coupling, and a P bracket on the outside to hold the prop parallel to water surface. Am I missing anything?

 

[skaraborgcraft]

You are trying to convert your 10lb or torque through a rubber boat. Plywood interlocking floors in infalatables is nothing new, but you will need to add some engine stringers the full length to stop the inflatable folding in on itself. You might think about a direct "longtail" shaft option, which would be easier than going through the floor, P bracket and rudder. You wont keep shaft bearings aligned in a rubber boat without creating a rigid structure to carry everything.

 

[gonzo]

Give it a removable hard bottom. I have great skills and capacity with wood working, though this time would be manufactured marine plywood. Speculating here: 3-4 separate pieces, 1 each for bow, transom, 2 in middle. Either a middle or transom piece would have 4 ‘bolts’ sticking vertically. ‘Bolts’= quick release latches akin to your bicycle seat height adjustment.

That will absolutely not work. The engine and shaft need to be aligned. You can install an inboard on a RIB because it has a fiberglass or aluminum hull. Also, a flat bottom will only work if you have a surface piercing propeller (surface drive). However, keeping the propeller at the proper depth in a soft boat will not be possible. Further, the power you have vs the weight is not going to give you 15mph. Inflatables have a lot of drag.

 

[montero]

That will absolutely not work. The engine and shaft need to be aligned. You can install an inboard on a RIB because it has a fiberglass or aluminum hull. Also, a flat bottom will only work if you have a surface piercing propeller (surface drive). However, keeping the propeller at the proper depth in a soft boat will not be possible. Further, the power you have vs the weight is not going to give you 15mph. Inflatables have a lot of drag.

Everything is possible technically speaking , for me I always think : bang for the bucks .I seem more easy adopt diesel powerhead to some outboard. Yanmar outboards are too heavy for this aplication . There was some Chinese attemps for cheap diesel outboard.

 

[gonzo]

Everything is possible technically speaking , for me I always think : bang for the bucks

That is not true technically speaking. There are limits to technology.

 

[BlueBell]

I claim to be handy with wood, yes.

Then you could follow a design building plan, one that is proven-good and meets your needs.
A stitch-and-glue would be a fast build. Or simply buy used that's already built.

What you're proposing; fitting the wrong engine to the wrong boat, is just plane silliness, IMHO.