| Author |
Message |
    
preventec47
New member
Username: preventec47
Post Number: 1
Registered: 08-2016
| | Posted on Tuesday, August 02, 2016 - 11:28 pm: | 
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I guess for the last 4 or 5 decades the personal watercraft have taken over all of the small vessel
activity so I have a 25hp Kohler VTwin that I want to put in a 10 or 11 ft long boat. I am thankful
to have found this website focusing on how they
did it long time ago and if they found ways that
would work reliably and economically I should probably try the same techniques. Other than GoKart rear axles with sprockets and automotive rear axles where a shaft extends from the differential to the wheel which is supported by
wheel bearings and the axle shaft has O-rings to keep the differential oil from leaking out... I have no other ideas as to how to design an inboard setup for a small boat and 25 hp Kohler. All ideas and suggestions would be greatly appreciated. |
preventec47
New member
Username: preventec47
Post Number: 2
Registered: 08-2016
| | Posted on Wednesday, August 03, 2016 - 07:05 pm: |
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The only people I can find practicing the use of inboard propulsion in small boats is the Smith Island Crab Skiff association and it looks like
they have dozens of guys who are racing their skiffs
using 20 hp VTwin lawn mower engines.
I am wondering maybe if when I build mine I should
stick the prop shaft through the transom and have
the main prop hub above the surface of the water
in a surface piercing propeller mode. |
preventec47
New member
Username: preventec47
Post Number: 3
Registered: 08-2016
| | Posted on Wednesday, August 03, 2016 - 07:07 pm: |
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Here is another picture of a Crab Skiff racer |
preventec47
Member
Username: preventec47
Post Number: 4
Registered: 08-2016
| | Posted on Wednesday, August 03, 2016 - 07:09 pm: |
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and another image.....
how are they building these things ? |
jb_castagnos
Senior Member
Username: jb_castagnos
Post Number: 1194
Registered: 07-2002
| | Posted on Wednesday, August 03, 2016 - 09:08 pm: |
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You will probably want a 1" shaft, stainless or some other corrosion resistant metal, especially in salt water. The shaft will have to be tapered and keyed, the end threaded. The strut to hold the prop can be bought or fabbed up, a piece of tubing to hold the cutlass bearing and plates welded, again stainless if in salt. The stuffing box may also be found on ebay, different designs depending on layout. You will need a flange for the propeller shaft and one for the motor, can be bought or machined. The surface prop will be experimental, you'll probably be on your own, but if you want it and have the time to experiment go for it. |
david_doyle
Senior Member
Username: david_doyle
Post Number: 68
Registered: 03-2013
| | Posted on Wednesday, August 03, 2016 - 10:15 pm: |
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"when I build mine I should
stick the prop shaft through the transom and have
the main prop hub above the surface of the water ?"
No, those guys are using crab skiffs/Chesapeake (sp) skiffs that have more then enough rocker to allow a prop to swing under them. Here is what, I assume they look like below the waterline (?)
http://i455.photobucket.com/albums/qq271/tilroh/Tempo%20Details/Tempoprimed_zps1 b33e3e7.jpg~original
I think in your area you would be able to sell that engine for someone's mud motor/surface drive project and shop instead for a proper Marine engine or if you are not content with displacement speed (gasp) one of those new fangled outboards. They make em in 25 hp and if they got points and ports they are a song to buy and whistle to fix. |
kayak
New member
Username: kayak
Post Number: 3
Registered: 06-2016
| | Posted on Thursday, August 04, 2016 - 09:23 am: |
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Rocker is:
http://www.paddling.net/guidelines/Images/bottoms_profile.gif |
ned_l
Advanced Member
Username: ned_l
Post Number: 39
Registered: 08-2012
| | Posted on Monday, August 22, 2016 - 12:22 pm: |
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If you put in anything close to the amount of rocker that is shown you will not be able to use about 23 of the 25HP that you are talking about. That much rocker is good for a row boat, but if you start to put power in a boat like that and increase the speed the transom will bury itself and the bow will point toward the sky and that's about it. I suspect those crabbing skiffs really have very little rocker and quite flat runs. |
ned_l
Advanced Member
Username: ned_l
Post Number: 40
Registered: 08-2012
| | Posted on Monday, August 22, 2016 - 01:10 pm: |
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In addition to what has been mentioned above (coupling, shaft, strut, prop) you will need a thrust bearing on the prop shaft. An inboard boat actually moves by the prop pushing on the shaft, the shaft pushing on the engine, and the engine 'dragging' the boat through the water by the engine beds. For this to happen, all the thrust of the prop shaft needs to be transferred to the engine or something else. An engine without a thrust bearing for the prop shaft will be quickly ruined. Crank shafts and main bearings are not designed for thrust. A thrust bearing needs to be installed on the shaft before the engine. (common with raceboats) |
preventec47
Member
Username: preventec47
Post Number: 5
Registered: 08-2016
| | Posted on Monday, August 22, 2016 - 01:52 pm: |
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I am a little surprised by the fact I have not been able
to simply copy other peoples designs but it
has been too difficult to get the details. I know
the basics and have been sketching ideas for solutions that are affordable.
So far to borrow components from Sail boats for approx 25 hp the
prop, shaft, strut, shaft log all cost on average
500 or 600 each. I look at a go kart and look
at the boat I want to build and I see smallish recreational
vehicles and I believe the propulsion
parts should be about the same cost. so I am noodling
with centrifugal clutches and go kart sprockets and rear axles and bearings etc.
Specifically for thrust, I would like to use std
auto wheel bearings at each end of the prop shaft
and maybe an automotive drive shaft U-joint or
a CV joint connect from a front wheel drive.
I agree the thrust needs to have a planned transfer
to the hull both in forward and reverse
directions ... I am not in an area to see lots
of inboards and hate knowing that all this has been
worked out by thousands over a hundred years ago.
I am also drawn to using 5 or 6 of these
RING SEALS http://www.mcmaster.com/#rotary-seals/=13p9eh5
in the shaft log just ahead of the propeller
and my plan is to squirt grease between each
ring seal by use a zirc fitting placed between each one on the shaft log. |
robert
Senior Member
Username: robert
Post Number: 732
Registered: 07-2003
| | Posted on Monday, August 22, 2016 - 06:32 pm: |
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All the info you need and much more is to be found in Ideal Series Volume 14, "The Construction, Care, Operation and Maintenance of Marine Engines", Published by "Motor Boating" (Magazine), 1st edition 1933, reprinted 1945.
http://forum.woodenboat.com/archive/index.php/t-14720.html} |
ned_l
Advanced Member
Username: ned_l
Post Number: 41
Registered: 08-2012
| | Posted on Tuesday, August 23, 2016 - 08:36 am: |
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Where are you located? There are cost effective ways to do what you want to, and some of what you are talking about is a bit misdirected. (standard U-joints and CV joints are not designed for thrust.) Standard shaft mount thrust bearings are available and not expensive. The two biggest reasons small inboards fell out of favor over the decades are that outboards became popular and 'inexpensive' (not so much today), and the parts to meet USCG requirements aren't so available anymore (flame arrestor for the carb., fuel tanks can not be gravity fed, etc).
There are lots of plans and good ideas available on how to do this, as well as some good knowledge on this site and others. |
ernie
Senior Member
Username: ernie
Post Number: 2304
Registered: 01-2002
| | Posted on Tuesday, August 23, 2016 - 09:16 am: |
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Here are a some pics from the Calvert, MD show of crab skiffs out of the water and a YouTube of them in action
click here
 |
david_doyle
Senior Member
Username: david_doyle
Post Number: 72
Registered: 03-2013
| | Posted on Tuesday, August 23, 2016 - 11:40 am: |
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It does look like they retain the sail/oar friendly aft end.
a 3-5 hp outboard clamped on the starboard gunnel aft of midships would be a nice way to move thru the water. |
ned_l
Advanced Member
Username: ned_l
Post Number: 42
Registered: 08-2012
| | Posted on Tuesday, August 23, 2016 - 02:55 pm: |
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They do have a good bit of rocker in the chine, but it is over such a long distance that it doesn't suck the bottom into the water. Fun looking boats. |
