shaft log for something like White's Rescue Minor

[John Carswell]

I am looking for advice on shaft log/bearings/seals. I am an amateur, an old man trying to learn something new.

I am building a boat inspired by Rob White's Rescue Minor. It is not as wide; the box keel is more like a rowing scull. A sliding seat rolls on tracks inside the box keel. It is intended to be a big rowboat with an auxillary inboard motor.

I am designing it as I go along. I understand that this is not the way experienced people build boats but I enjoy it.

The form is built, stem and box keel are together; now I have to figure out how to put a sternpost in the box keel that will marry the framing for the tunnel-stern to the box keel.

The horizontal propeller shaft will pass through this sternpost. I need to design the piece around the log, bearings, seals, etc. and I don't know how to specify these parts.

Robb White said it was the easiest shaft log to install and he referred to the cutless bearing.

I am designing around the BetaMarine 13.5 HP diesel.

Here are my questions for anyone with the inclination to point me in the right direction.

1) It is my understanding that a cutless bearing is water-lubricated. Will such a bearing serve well if it is mounted at the waterline?

2) Does it need to be open to the sea at both ends?

3) The cutless bearing in my old Correct Craft ski boat was mounted in a strut. Is there some standard bronze hardware that would be mounted inside the shaft log to be drilled through the sternpost, bolted from the outside that would carry a cutless bearing?

4) Following what I could glean from Robb White, the propeller shaft is to be horizontally mounted in inboard pillar blocks with the forward end of the shaft in a thrust bearing. A sheave would be mounted on the shaft between the pillar blocks to be turned by a belt from a motor mounted above. Does it need another bearing outboard of the shaft seal? The same question another way -- how much unsupported 1" shaft between the bearing and a 12" propeller is practical?

5) I would like to avoid the drip of a packing gland. Is this an application where a dripless shaft seal would work?

6) Are oil-bath shaft bearings worth considering.

7) I spent all day Googling and felt more ignorant the more I saw. Is there a favored vendor with a patient staff that would help an old man buy the right stuff?

[redviking]

John,

I have been following Aitkins' Rescue Minor for a while with great interest. I am looking to build one myself in the not to distant future. I can give you some answers to your questions. Hopefully you can get some other contributions as well to improve the discussion and help you make some decision.

1) A cutlass bearing is forced water cooled and can be used on the waterline. I live in Australia and use an American brand called Duramax. The bearing will need to be force feed from the raw water overboard that comes out of the heat exchanger of the engine before going over the side

2)The bearing is open at the propeller end and sealed at the gearbox end. Use a Duramax or PSS seal at the inboard end. If you search the internet for either Duramax or PSS you willl easily find them. Both are companies based in the US and will have distributors all over.

3)Your old vessel probably did not have a keel as such and the shaft went through the hull and needed to be supported near the propeller hence the strut. Since the "Rescue Minor" has a keel, the bearing is mounted in the keel. No need to put any sought of fitted on the keel.

4)The means of changing direction of the shaft that Robb White mentions is great if you want to save a few quid (sorry bucks) and like to be seen to be a bit innovative. However if you can afford it buy a small gearbox to mount on the back of the engine. I am not familiar with the engine you are going to install, but if you talk with the supplier and tell him what you propose to do he will be able to help you out. These guys can be a fountain of knowledge as that is what they do, match engines to boats. With regards to the shaft bearings, depending on the length of the "shaft log" in the keel you will only need one bearing. An idea that I have seen used is to glue a stainless steel tube into the shaft log and mount the bearing inside the tube. That way when you need to change the bearing, simply knock out the bearing. Also the seal and be fitted to the end of the tube on the inside. The distance that the shaft will protude from the back of the bearing before the front of the propeller boss is 1". A rule of thumb is the diameter of the shaft is the amount that the shaft will protrude.

Hope this helps. I am a Marine Engineer by trade and currently oversee large aluminium ferry rebuilds.

Eric

[redviking]

John,

I forgot to answer the last couple of questions.

5) try a duramax or pss seal as I previuosly pointed out. The are designed for this application.

6) don't consider a oil bath white metal bearing. Do expensive and just not for this application.

Try to get hold of any books by Dave Gerr or Ted Brewer about props, shafts, engines, systems, boat design etc. I have a couple at home which are good basic reference books for the amateur. They are written in a style that is easily understood. Try Amazon or book depot. Will set you back less than $20 per volume. They are well worth the investment.

Best of luck with your project. I look forward to seeing the progress on this forum.

Eric (Brisbane, Australia)