1992 Sea Ray Sundancer 290 Outboard Conversion Project

CLB said:
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/273488770875

You can thank me later :D
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Spot on, that'll do it. It's got a nice long leg which is useful and it's a 2 stroke which makes it less desirable [perfect for you though as they are a well sorted direct injection motor]

Now you need a custom fabricated 'Gill bracket' which could be made by any good fabricator in ally plate then anodised/powdercoated.

Looks like you could be sorted for about £6k [£10-£12k] total expenditure which feels more sensible for the mongrel you are creating.
 
thesaintlyone said:
Sounds good to me ��
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I doubt 200hp will be enough, suspect it will just climb the hump and push a huge wall of water at about 12knts, that is not a small boat.
Its a planing hull so it will do up to about 7/8knts in displacement mode with minimal hp of say 100hp which would be perfect with some margin to push a tide. But it cannot operate at speeds of 8-15knts until it has climbed over the hump and is fully planing and that will require 300hp. There are plenty of example of similar boats that are under powered with about 200hp and will not plane with more than two people on board, full fuel etc... e.g Sealine S28 with a KAD42 (230hp diesel). 200hp will not be enough.
 
Not sure about the Yamaha 250 HPDI motor, they used to burn holes in pistons when they came out, not sure if Yamaha ever resolved this one - tread carefully!
 
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Chris_d said:
I doubt 200hp will be enough, suspect it will just climb the hump and push a huge wall of water at about 12knts, that is not a small boat.
Its a planing hull so it will do up to about 7/8knts in displacement mode with minimal hp of say 100hp which would be perfect with some margin to push a tide. But it cannot operate at speeds of 8-15knts until it has climbed over the hump and is fully planing and that will require 300hp. There are plenty of example of similar boats that are under powered with about 200hp and will not plane with more than two people on board, full fuel etc... e.g Sealine S28 with a KAD42 (230hp diesel). 200hp will not be enough.
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Plus a diesel engine has more torque - I suppose if he changed the prop to a much finer one (effectively running in a lower gear) he would lose top speed, but generate more power to get on to the plane. It will be an interesting experiment.
 
Grubble said:
Plus a diesel engine has more torque - I suppose if he changed the prop to a much finer one (effectively running in a lower gear) he would lose top speed, but generate more power to get on to the plane. It will be an interesting experiment.
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The engine is 250hp ?
 
thesaintlyone said:
The engine is 250hp ��
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yes but where does it develop it's torque. If it develops it's torque outside of the displacement to planing transition band you may never see the motor delivering all those ponies. If it helps think of a car in top gear trying to take off uphill. If you then underprop it to get the torque (i.e. 2nd gear) you will also not see all those ponies put to use plus not go very fast. i.e. on the plane maybe but not much more in it.
 
BruceK said:
yes but where does it develop it's torque. If it develops it's torque outside of the displacement to planing transition band you may never see the motor delivering all those ponies. If it helps think of a car in top gear trying to take off uphill. If you then underprop it to get the torque (i.e. 2nd gear) you will also not see all those ponies put to use plus not go very fast. i.e. on the plane maybe but not much more in it.
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OKi my thought is this. 50hp will move the boat right so 250hp should move it pretty well. So if I have a pod/bracket designed that would accommodate say a single 350 I can start with the 250hp and if the need suits upgrade to something bigger or more efficient but at least I have a starting point now ?
 
I am not convinced you will find a 50hp will be any different to my initial comments unless you can also find a high thrust low pitch prop for it. In bygone years I had a 8hp 2stroke as an emergency kicker engine to a single on a 18 foot cuddy cabin. After 1/3rd throttle I just lugged the engine but it could push me to 4 knts so was left as is. As in my comments in the other thread. It's complicated and could turn out to be a costly exercise is trial and error. I still maintain sticking to known parameters is the best way forward and that was that single 300 V8 as the way forward. However I wish you luck and am following with interest.
 
'will likely be river cruises up and down the Medway as far as Stangate which is a great little overnight creek just before the Medway reaches the Thames estuary and longer cruises in the Thames estuary and up to the far reaches of the Thames its self'

Speed limits according to Google:

The speed limit above Wandsworth Bridge is 8 knots. Between Wandsworth Bridge and Margaretness Point (roughly three miles east of the Thames Barrier), it is 12 knots.

why does the boat need 300 Hp ?
 
simonfraser said:
'will likely be river cruises up and down the Medway as far as Stangate which is a great little overnight creek just before the Medway reaches the Thames estuary and longer cruises in the Thames estuary and up to the far reaches of the Thames its self'

Speed limits according to Google:

The speed limit above Wandsworth Bridge is 8 knots. Between Wandsworth Bridge and Margaretness Point (roughly three miles east of the Thames Barrier), it is 12 knots.

why does the boat need 300 Hp ?
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If he wants it to plane. If he doesnt I'd still be looking for a single inboard diesel engine, maybe a Volvo Penta D2-50. But as they are going to cost the same as a single 300 V8 petrol I think it's a non starter. There are of course other second hand diesel engines in the 50hp range that may be got significantly cheaper
 
simonfraser said:
'will likely be river cruises up and down the Medway as far as Stangate which is a great little overnight creek just before the Medway reaches the Thames estuary and longer cruises in the Thames estuary and up to the far reaches of the Thames its self'

Speed limits according to Google:

The speed limit above Wandsworth Bridge is 8 knots. Between Wandsworth Bridge and Margaretness Point (roughly three miles east of the Thames Barrier), it is 12 knots.

why does the boat need 300 Hp ?
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For the bit between the barrier and the start of the Medway 6 knot limit, which is quite a way in from the entrance :D
 
thesaintlyone said:
OKi my thought is this. 50hp will move the boat right so 250hp should move it pretty well. So if I have a pod/bracket designed that would accommodate say a single 350 I can start with the 250hp and if the need suits upgrade to something bigger or more efficient but at least I have a starting point now ?
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No that is not at all correct.

You have a planning hull

That will have a hull speed ( the speed it can just slip through the water in effect) of about 7 knots. This will require relatively little power.

To go faster than this you need to get on the plane. This requires a lot of power. You are literally lifting the weight of the boat above the water so that only the rear part is the water and it can in a manner of speaking skim over the top.

The engine has to get you there, and then it can slack off a bit as keeping it there requires less power.

So just as 50hp will move it, 250hp will only move it faster if it is capable of getting it on the plane. If not you will just burn fuel as the bow get higher and higher.

Try it on a two engine boat. Use one engine and see what happens. Generally ( and I am sure you can find an exception somewhere) you will probably get 10 knots ish. The "test" is not wholly accurate because of drag, weight etc but it basically proves the point.

The power of an engine is different at the powerhead ( what is quoted) vs power at the prop. I have read some articles that suggest for an outboard this is as much as a 50% degradation.

There have bene prior threads for example http://www.ybw.com/forums/archive/index.php/t-139509.html

I would caution against seat of the pants stuff. You need to determine for the boat you have how much power at the prop the boat requires to perform as you wish. You need to know with 100% certainty that it will plane, as if not the project becomes pointless. Going around at displacement speed in a planning hull is not a nice experience in anything other than totally flat water.

The OEM installation is a great start. That was on outdrives and someone on here will know the approx engine power still at the prop for such an installation. Next what is if for an outboard and this will give you some degree of certainty that the boat will actually perform. Bear in mind power that to get a boat on the plane if it is struggling ( tenders) you move weight forwards. Here you have moved it backwards ... so the power at the prop the OEM setup had should be treated as a minimum.

There needs to be a degree of technical rigour here or you will end up with something that you cant sell ( not your aim ) but that also does not deliver what you want ( which is you aim).

Thoughts of doing this twice should vanish in my view. You are on a budget. Do It once.

At the end of this reverting to one of the OEM specs ( single or dual engine) may well give you what you want AND result in something someone else will buy which seems a win win all round and the is a huge reduction in risk.
 
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