20th February 2023 - The issue of Thrust Blocks
An important point to remember when adding an engine and drive train to a boat is that the thrust from the propeller has to be transferred to the boats hull at some point. Failure of the technology designed to handle this can lead to some amusing accidents. The classic such accident is probably when the engine is put into reverse and the propellor proceeds to withdraw the shaft down the the stern tube. However, such accidents are probably best avoided and solving the problem does not require too much extra technology.
The classic Thrust Block - basically the thrust block is a flange on the drive shaft that runs on a bearing that is at right angles to the line of the shaft. There needs to be a bearing on each side of the flange since it is assumed that the engine will drive it both forwards and in reverse. A beautifully made example is shown in the photos and drawing below and was created for the steam launch Iron Chief. (Details to follow)
The DIY Approach Thust Block Substitute - in this approach the idea is to keep the cost low and to make use of equipment that exists or will have to exist in the installation. Thus for taking the forward thrust down the drive shaft it makes sense to carry it forward to the engine. That will be securely mounted to the engine beds so ought to be capable of transmitting to the boat the amount of energy that the engine ouputs. However the typical ST 5 Cygnet engine or other small canoe engine is not designed to carry a latteral load down the drive shaft. It is likely, though, that the flywheel has flat faces machined on the hub and that the bearing assembly that carries the output shaft has a flat face also. Into that gap between the flywheel and the engine frame it is possible to fit a thrust bearing. The shaft of the Cygnet is specified at 5/8" (15.88mm) and roller and ball thrust bearings with washers to fit that size are widely available. For example:
eBay - a TC1018 roller bearing with washers retails at £4.81 (for 2) with free postage.
Simply bearings - 5/8" imperial bearing with two washers for £6.41+VAT with free postage.
The working gap between the flywheel and the engine needs to be set at approximately 3.6mm which does not allow too much room for arranging for lubrication. It is possible that a diagonal hole through the flywheel coming out on the bearing surface might be a solution for squirting either oil or grease, along with a ring of fine holes in the washer to allow the grease to reach the rollers.
Whilst the above solution should work for forward propeller thrust, the next question is how to deal with reverse thrust.
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