Sterndrive conversion

[Dieselboatman]

Hello all

I am a diesel mechanic by trade and a hobby fabricator in my spare time

I am looking at getting a project boat, but dont want an outboard

Ideally inboard but sterndrive possible

However what puts me off sterndrive (and outboards) is the cost of them

Seen some inboard boats in size I would like (18~22ft)

I rather like the colvic 20 and seaworkers personally


However, I see an awful lot of sterndrive boats available (either with no sterndrive or an old knackered one still fitted)


Would it be possible to fabricate a plate or even a box to fit where the stern drive would, and then simply have a standard inboard gearbox and a stern gland instead?
With a prop behind the boat instead of under, and then just fabricate a rear mounted rudder?

Would this work? Any issues with doing this?
Just wondered why all of the inboard boats I have seen have prop underneath and not at back (in similar location then to a sterndrive or outboard)

Any advice/assistance would be great!

 

[Paulg25]

Not an expert but I think the prop needs to be lower in the water than it would be with your idea.

 

[CLB]

Prop won't be deep enough in the water, assuming a planing hull. What you refer to is more akin to a surface drive, which needs special props among other things.

 

[kashurst]

Would it be possible to fabricate a plate or even a box to fit where the stern drive would, and then simply have a standard inboard gearbox and a stern gland instead? Yes - subject to the hull being strong enough. Have a look at Radice Helice website for ideas of fittings available. You can buy/make fittings to allow you to put a shaft through the bottom of the hull and fit a P bracket to support it.
(from Radice Helice website: Semi-rigid stuffing box P.S.O. revolving type: Premitreccia semirigido orientabile P.S.O.)

With a prop behind the boat instead of under, and then just fabricate a rear mounted rudder? If you just put the shaft straight out through the transom of a planing boat it won't be very efficient and probably noisy because the hull will block much of the water flow. What you describe works on canalboats etc but the hull shape is very different.

Would this work? Any issues with doing this?
Just wondered why all of the inboard boats I have seen have prop underneath and not at back (in similar location then to a sterndrive or outboard)

 

[Kawasaki]

If I found a 'project' with a sterndrive
I'd convert to an out board!
Each to His own it seems
I really do admire people who take projects on though
We all must have read Wayne's threads/posts on His two
Check those out Dieselboatman
Insperational!

 

[Dieselboatman]

Thanks for the replies

I will have a look into the surface drive and the radice helice stuff

As for converting to an outboard, I know that's common, but they cost a lot of money being a single self contained unit

Being a diesel mechanic, a marinised 2.5di or even 1.8d (fsd425 and xld respectively) are easy acquisitions, and I can purchase parts separately so although total cost may add up to similar eventually, it's not all In one hit

I am considering with an onboard to go down the route of dry stack exhaust, plus either keel cooling or heat exchanger (would like to avoid raw water cooling as much as possible)

 

[Momac]

Hello all

I am a diesel mechanic by trade and a hobby fabricator in my spare time

Just wondered why all of the inboard boats I have seen have prop underneath and not at back (in similar location then to a sterndrive or outboard)

Look at the way the prop shaft is supported on a normal mid engined shaft drive boat
Also with shaft drive the rudders are astern of the prop
Not possible with the position of the engine in a sterndrive ....... unless ..............you turn the engine around - have zed drive arrangement .
Probably much cheaper to rebuild the outdrive.

 

[Chris_d]

Propeller needs clear water in front of the blades otherwise it cannot generate thrust, it does this by screwing thru the water. If the prop is against the flat transom it won't work, at best it will be an inefficient paddle wheel and just move the Stern sideways.

 

[Dieselboatman]

I did wonder about height of prop being an issue....

Ok I will just keep looking for a colvic seaworker or 20

Really would prefer an inboard boat, and happy with a displacement hull, 6~7knots is plenty, just want to have a roomy boat to go a few miles off coast for fishing and day out

Not seen any semi cathedral hull with an inboard

 

[rotrax]

What about a knackered high output outboard with a good leg/transmission and mount a diesel on top instead of the petrol donk.

Need a bit of thinking about mind................................

 

[burgundyben]

The old adage "in every boatyard there is an outdrive in bits'

 

[Rappey]

Another issue with a surface drive can be getting going or getting on the plane.
Once achieved they become very fast.
A water jet drive could be another option? I had the use of a 28ft cathedral hull boat powered by 220 hp. It was deafning as diesel needed high rpm to go fast and was a total pig at slow speed.
Outboards are expensive but very reliable, take up no space and are very quiet.

 

[DougOut]

I've often dreamed about similar but on waking up realise I've not got the skills or temprament to undertake such a project! However, trying to think out of the box, could a V drive be an option? or Hydraulic drive? Maybe even electric. Lots of batteries and just use the engine as a generator. Very 21st century!

 

[scottie]

Another issue with a surface drive can be getting going or getting on the plane.
Once achieved they become very fast.
A water jet drive could be another option? I had the use of a 28ft cathedral hull boat powered by 220 hp. It was deafning as diesel needed high rpm to go fast and was a total pig at slow speed.
Outboards are expensive but very reliable, take up no space and are very quiet.
[/https://www.tripadvisor.co.uk/Attraction_Review-g154998-d1431008-Reviews-Whirlpool_Jet_Boat_Tours-Niagara_Falls_Ontario.html
Something like this?

 

[Kawasaki]

Ok I will just keep looking for a colvic seaworker or 20

Really would prefer an inboard boat, and happy with a displacement hull, 6~7knots is plenty, just want to have a roomy boat to go a few miles off coast for fishing and day out

If you are going down that Route look at a Channel Island 22, better than a Seaworker
I have had both

 

[vas]

2-t petrol jetski engine with drive ? takes no space, but it's petrol.
My 3m avon jetrib has a yamaha 70hp engine, it's scary fast at wot

 

[Dieselboatman]

I will have a look around with some of these ideas!

As for channel island 22, they do look nice but haven't seen any for sale in budget

I like the couple of colvic I've seen as the wheelhouse is further forward for more deck space (less cabin in front of wheelhouse but for a day trip boat as long as 2 kids will fit it's fine!

 

[Dino]

The other option is to buy a small petrol engined sterndrive boat and fit a diesel. Motorboatowner online magazine are refitting a petrol Fairline Vixen and doing a rebuild on the engine and drive. Might be a nice bit of reading as all the back issues are available online. Www.motorboatowner.co.uk
Another option is to refit a twin petrol sterndrive boat like a Princess 25 with a large single diesel with modern out drive

 

[Dieselboatman]

I am just concerned that a sterndrive is going to cost a fortune as it's a full unit (similar to outboard)

An inboard setup is what I would really like just for a more simple setup in some respects

I did look at a Tod Tuna last year, complete boat with twin 50hp Ford petrols, gearboxes were good but engines were knackered
Plan was to swap those for a pair of transit diesels (60~65hp) or even more if turbocharged fitted

Basically, I would like either a displacement or semi displacement hull, 18~22ft, with a wheelhouse closer to the bow for more room

I keep seeing the microplus, flyers, and various other dory hull boats, but I'm living down in cornwall, and even in the estuarys a semi cathedral hull slaps about on a calm day

And speaking to a few locals, apart from a few, they all seem to have changed to semi displacement hulls if they are venturing off shore more than a mile

Launching is no issue luckily in cornwall, with a huge number of places to launch even bigger boats

But biggest issue, is a limited budget each month

I am more than happy (happiest infact) getting an almost bare hull project, and then each month purchasing what I need

Anyone got a solid bare hull they dont want???

 

[Kawasaki]

Yep
Semi displacement for your area and what you intend doing