Robb White



Boat Photos

Sport Boat

Rescue Minor,

Rescue Minor
Machinery Details

Gill Net, story

Wakulla County Model
Bow Skiff, story

Last Goose Hunt, story


Rescue Minor Machinery Details

(This is a compilation selected from a bunch of articles which appeared in Messing about in Boats .... you really ought to spring for the $28 to subscribe for 24 issues.)

I don't like the word "marine" when used as a adjective ahead of any noun. Marine anything means excess money and, to my notion, excess money results in the proliferation of nonsense... sort of like the New Orleans project. I think marine engines as built in this country for recreational use are horrible examples of too much engineering leeway enabled by the excess money so willingly shoveled out by the "boating public." Heat exchangers, rubber impeller raw water pumps and the ridiculous "inboard outboard" are the best examples of what I mean. To me, a real marine engine is antifreeze cooled by an automotive style centrifugal pump circulating coolant through a keel cooler just like every commercial boat built to work for its living in salt water in the world. Water cooled manifolds are necessary on big engines, and an oil cooler for both the transmission and engine are necessary, too, but on such a rig as the Rescue Minor, there is not enough engine room heat generated that those things can't be air cooled like in a tractor or automobile.

My engine is a three cylinder Kubota basic engine of 722cc (including alternator, starter, water pump and fuel system... no bell housing or clutch.... just a naked flywheel on the stern). A legitimately "marinized" engine like it costs something like $5,000. I paid $1,300 on a "special" Kubota was having and I think the going rate for the same engine is up around $2,500 directly from Kubota now but they come up in surplus style situations every now and then. A friend of mine bought two of them for around $1,400 each. It pays to look around. Ebay is a good place to start.

My transmission set up is the most peculiar part. I have done that belt drive business before on sailboat installations. On those I used regular 'V' belts with one of those adjustable pitch sheaves so I could get the ratio exactly right for the propeller. Those were low horsepower rigs (usually with the Kubota 4hp Diesel engine they use to power the generator on those flashing arrow signs you see along the interstate) and we just rigged it so the flexible motor mounts could tension the belt. To get neutral, we pulled down on the engine to slack the belt and reverse was a "Troy Bilt" tiller rubber tired reverse disc which engaged another groove on the shaft sheave when the engine was depressed all the way. The whole thing was a copy of the Troy Bilt system which works real well on a tiller and did on a boat, too. The setup was silent and the engine could be suspended in gimpy enough motor mounts to eliminate vibration completely. The shaft ran in two pillow blocks (or flange blocks) so it always stayed aligned and the seal we used was a standard ceramic/graphite industrial pump seal which never leaked.

I did the same thing with the Rescue Minor except I used one of those poly "V" belts because they'll transmit more horsepower with less weight and space. I depressed the stern of the engine by twisting a link of chain connecting the stern of the engine to the bottom of the boat by stomping on a pedal sticking out of the front of the engine box. It worked alright. It took the full stroke of the pedal (maybe 60°) go from forward to reverse and neutral was a little creepish but I could have cured that by making some belt guides to keep the belt completely clear of the drive sheave when slack but I never got around to it. The rubber reverse wheel is mounted directly to the drive pulley. It is nothing more than a Snapper riding mower propulsion drive wheel. The reverse disc is made of phenolic and is mounted directly onto the driven pulley. Though it required a good bit of pedal pressure to keep from slipping in reverse, it was possible to reverse the prop while running wide open.... going from wide open forward to wide open reverse with no cavitation... sort of awesome.

Belt tension was adjusted by raising up on two hinges upon which the soft rear motor mounts were mounted. I had bolts I screwed down to open the hinge to raise the motor mounts. It worked well but I got to worrying about all the overhanging load on the crankshaft from the belt tension required to keep the belt from slipping at maximum output so I built a complicated compressed air belt tensioner. It worked by pushing two bearings apart, one mounted on the propeller shaft and the other aft of the sheave on the stub shaft mounted on the flywheel. This put the pressure nearer the pulleys and also completely relieved the crankshaft of the engine from having to tension the belt. That contraption worked by an air spring. There were two ways to get air to it: One was a little electric compressor and regulator and the other was just a regular tire valve and bicycle pump. Though that relieved my worries about the crankshaft and tensioned the belt plenty tight for the 20 hp the Kubota will actually put out, it was hard to depress the clutch/reverse pedal while the belt was under full tension. I put a quick-relief valve in the line which released air pressure on the tensioning device. It worked well. With the air off, the boat would run up to about ten and a half knots without the belt squealing and the clutch/reverse worked easy and, with a flip of the valve, I had full tension.

So, to continue this drama, after about a thousand hours the damned pneumatic tensioning apparatus wore out from wiggling with the engine on its soft mounts and the thing went to rattling most aggravatingly back there. We use the boat for normal transportation all the time and, though I had some more wild ideas, I quickly made a simple little mechanical belt tensioner out of a semi truck engine valve spring and slapped it in there to push the two bearings apart and it has been working for many hours without problems in our particular application (which is run the boat in gear all the time). I can still work the pedal for neutral and reverse but it is hard and I never do it. I have some plans, though.... when I get around to it.

The reason I decided on air to tension the belt was that I have worked on industrial furniture factory machinery for a long time and like the way air-actuated mechanisms work. Besides, I initially planned to make the Rescue Minor so that it was completely independent of electricity in any form. I planned to adapt an engine driven compressor to the Kubota and modify the starter Bendix to run on an air motor. Though the engine is glow plug style, it will instantly start without them at any temperature above 40o (F) and I am sure that it would air-start without glow plugs at almost any temperature. Anyway, that was the plan and I still intend to do it. I will also modify the belt situation so the propeller shaft is held on both sides of the pulley by two pillow blocks attached to the bottom of the boat. That will make the propeller shaft stiff enough to handle the belt tension without anything between it and the engine but the belt. I plan to rig the air spring to raise the stub shaft of the engine with a lever off to the side and fix an automatic release valve so, when I push on the pedal for neutral or reverse, the tension pressure is relieved. I figure that I'll adjust the motor mounts so just a touch on the pedal will release the air and slack the belt into neutral and it will idle like that until I depress the pedal to get reverse or kick it up to close the valve and re-tension the belt.

Cooling System

The engine is cooled by a keel cooler which is just a 1/4" thick by three feet long by three inch wide copper plate with a "U" of one inch copper pipe brazed to it. It is set flush with the bottom of the boat in a fiberglass box which sticks up above the bottom inside a little bit. There is no seal around the plate so water is free to circulate inside the box. Indeed, the cooling water for the wet exhaust is drawn from that box which removes any air trapped inside there and also provides a little circulation. That six feet of one inch tubing is plenty enough to cool the engine. There is a copper day tank holding about 1 1/2" gallons (US) of coolant and a copper expansion tank holding an additional half gallon. The day tank receives water directly from the keel cooler and is always cool enough to touch. The running temperature of the engine is regulated by the stock Kubota thermostat. I am sure the cool day tank inside the engine room helps keep the temperature in there down.

The exhaust water is pumped out of the box by a standard agricultural (not "marine" but it is the same thing only cheaper) "Surflo" all plastic diaphragm pump which I adapted to run directly off the power take off provided by Kubota on the forward end of the camshaft of the engine. I machined a metal plate to replace the electric motor which came with the pump so that it mated with the camshaft end. I have also done that before and such an installation has proven to be trouble free. Unlike a rubber impeller pump (Jabsco, Johnson, Sherwood, etc) the pump will run dry indefinitely without any damage to itself and without stopping up the water passages with pieces of rubber shed from the impeller when it self destructed. The pump will self-prime without having to be below the waterline. I have fitted diaphragm pumps on quite a few engines including Perkins, Yanmar, Volvo and Albin engines which proves that US Americans aren't the only people who can't figure out how to pump salt water reliably. I believe the rubber impeller pump was invented by marine engineers as a way to sell more marine engines. The only thing wrong with the Surflo is that it doesn't like sand and seaweed so I fitted a good sized strainer. Another agricultural pump in common use uses plastic rollers working in an eccentric chamber and they say they'll pump a slurry of granular fertilizer but they cost much more than the $60 Surflo.

The stock Kubota cast iron manifold is cooled by wraps of copper tubing into which the Surflo pump discharges. Those wraps of tubing are covered by a wrapping of thick copper foil secured by wrappings of copper wire. After the water passes through the tubing it goes into the exhaust right below the flange. Because the engine is mounted above the waterline there is no need for an exhaust riser. The muffler is a standard fiberglass "Vetus" 1 5/8" hose style. It works well, the boat is very quiet and the exhaust manifold does not heat up the engine room.

Because the engine is mounted with its crankshaft center above the shaft center, there is sufficient room under the oil pan to drain the oil out into a container for changing. That space allows for circulation of air around the oil pan to cool the oil without a water circulating oil cooler. The oil temperature of the Kubota stays very cool even after hours of extended running. In compliance with regulations I have an electric blower (standard 4-3/4" "muffin fan") drawing air into the engine room through a small dorade under the port side seat. This air blows directly onto the oil pan and then up around the engine to exit at a little outlet dorade on the after part of the top of the engine box. The air coming out is warm but not hot. As a matter of fact, when the blower burns up from sucking salt spray, the engine room stays cool. The only way you can tell that happened is that the outlet dorade starts sucking, too. I guess 722 cc worth of engine aspiration moves a lot of air at about 3,000 rpm.

I really like the whole rig. It is the lightest, quietest, most vibration free Diesel installation I have ever seen. I have some plans to make a few additional modifications beyond plans to set the clutch/reverse crap back up again but none to make any real big changes. One thing I am going to do is to extend the fiberglass stern tube all the way back to the propeller so as to sheathe all that naked shaft Atkin specified (he does that consistently on all his boats). I believe a naked shaft acts as a centrifugal pump and that not only drags uselessly on the engine but the movement of water in a non-linear way might try to slow the boat. You know if you stop up the inlet or outlet of a centrifugal pump, it is easier to drive than when it is pumping. I know that shaft will wrap up fishing line and stuff. I might try to tap into the stern tube to see if the turning shaft will pump enough water to cool the exhaust. It will be worth an experiment. I also want to sound proof the engine room a little. It isn't intolerably loud but it could be a little bit quieter.


The advantages to all this are: The drive mechanism is very efficient. Nothing gets hot like a gear driven transmission. The whole propulsion system is isolated from the boat by rubber... shaft, propeller, engine... so there is no noise transmitted mechanically. Because of that, it is possible for all the machinery to be mounted in very soft mounts so vibration is minimal. Belts are very tolerant of misalignment so all that wiggling is OK. At idle, the engine acts about like a chicken adjusting herself in the nest. The best thing is that all that is good old reliable industrial or agricultural machinery... no built in "marine" expense or treachery. See what I have been trying to tell you?


Robb White, boatbuilder and writer, Thomasville, Georgia

Designers and Builders of Custom Small Boats Since 1961
P.O. Box 561, Thomasville, GA 31799
Copyright 2004 - 2006 byRobb White.  All rights reserved.