1992 Sea Ray Sundancer 290 Outboard Conversion Project

[CLB]

People are getting hung up on what the boat had originally. A typical 29ft sportsboat will get on the plane with 250hp as long as the torque is in the right place and the prop is well matched. It may not make 35 knots, like it did originally, but I suspect it would still achieve 25 without too much trouble. The two stroke motor I pointed to earlier would be better than a four stroke due to having better torque characteristics.

 

[jrudge]

People are getting hung up on what the boat had originally. A typical 29ft sportsboat will get on the plane with 250hp as long as the torque is in the right place and the prop is well matched. It may not make 35 knots, like it did originally, but I suspect it would still achieve 25 without too much trouble. The two stroke motor I pointed to earlier would be better than a four stroke due to having better torque characteristics.

Given the financial outlay of the OP I would rather rely on something more than " I suspect" when deciding engines!

 

[LBRodders]

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 ��

Still not sure what the point of this project is? With every post, you seem to change direction.
350 horses in an outboard is serious money. So is 250, plus as many have said 'it aint enough'.

IMO, your best bet is a couple of circa 50-75s. They probably wouldn't have to match. Readily available and cheap. You wont plane but strap them on and you'll get about the river fine.

 

[CLB]

Given the financial outlay of the OP I would rather rely on something more than " I suspect" when deciding engines!

Well, it's an educated suspect, rather than a blind guess, and at £4K there is little to lose as the motor could be sold for the same. If a Sealine S28 or targa 28 can hit 30 knots with 300hp spread over two engines, there is little doubt that a very similar searay 290 would be able to get over the hump with a single 250, especially a two stroke. Reduced weight and drag should see the gap narrow quite a bit. If it was my project, and my money, I would have no hesitation in trying. You are unlikley to get anyone saying anything more definite than that, unless you can find someone that has put a 250 Yamaha hdpi on a sea Ray sundancer 290 before.

 

[thesaintlyone]

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.

I now have a base to design and build the pod from and I'm in no rush to sort the engine yet till the pod is built

 

[jrudge]

I now have a base to design and build the pod from and I'm in no rush to sort the engine yet till the pod is built

However the outboard support ( which I assume is the pod) will be radically different in design for 100hp ( 2x50 ) vs 250 - 350 hp.

 

[thesaintlyone]

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.

Whilst I have no problem with what your saying when I suggested converting to a Single inboard engine I received alot of grief for it even tho beyond the required Fibreglass work to re-do the Stern drive holes would work out fairly economical definitely more than finding two engines and two Stern drives

 

[thesaintlyone]

However the outboard support ( which I assume is the pod) will be radically different in design for 100hp ( 2x50 ) vs 250 - 350 hp.

Sorry I meant that Base would be for a single 300hp.

 

[jrudge]

Whilst I have no problem with what your saying when I suggested converting to a Single inboard engine I received alot of grief for it even tho beyond the required Fibreglass work to re-do the Stern drive holes would work out fairly economical definitely more than finding two engines and two Stern drives

You will get grief any which way really given most on here would not tackle either option. You want to do it and have asked for help so we are trying to help! You need something at the end of the day that is usable. Displacement cruising on a Planning hull is not the way to go. It is miserable unless on a totally flat sea.

So it must plane.

If you are going to do that you need to know that your mod will work before you start or you are wasting both time and money.

You have two options. Cheap and cheerful or do it right. I suspect the outboard route done right ( dual engines etc) will be little different cost wise to inboards which have the huge benefit that it will be worth something. however you don't want to do that!

So I suppose you are left with cheap and cheerful. Fine a cheap way to fabricate a bracket, ( £3k seems silly in the context of the overall project) buy the outboard found by CLB and hope of gives you a runabout that will plane and you have pleasure from.

Will it plane? I have no idea, but CLB reckons it will! £4k for an outboard, £1k for a fabricated bracket. Test it. Works, great, tidy it up and do it all nicely. Doesn't - sell the outboard on eBay. Cost £1k and some effort.

 

[thesaintlyone]

Still not sure what the point of this project is? With every post, you seem to change direction.
350 horses in an outboard is serious money. So is 250, plus as many have said 'it aint enough'.

IMO, your best bet is a couple of circa 50-75s. They probably wouldn't have to match. Readily available and cheap. You wont plane but strap them on and you'll get about the river fine.

Not changing direction but rather evolving. The boat will be on the Medway at Wateringbury and the natural tenancy is to use the boat for Cruising upto Riverside pubs restaurants etc and on occasion further trips up to the Thames estuary and into London. Pretty standard stuff methinks. I don't need massive power but don't want to be grossly underpowered at the same time ?

 

[thesaintlyone]

You will get grief any which way really given most on here would not tackle either option. You want to do it and have asked for help so we are trying to help! You need something at the end of the day that is usable. Displacement cruising on a Planning hull is not the way to go. It is miserable unless on a totally flat sea.

So it must plane.

If you are going to do that you need to know that your mod will work before you start or you are wasting both time and money.

You have two options. Cheap and cheerful or do it right. I suspect the outboard route done right ( dual engines etc) will be little different cost wise to inboards which have the huge benefit that it will be worth something. however you don't want to do that!

So I suppose you are left with cheap and cheerful. Fine a cheap way to fabricate a bracket, ( £3k seems silly in the context of the overall project) buy the outboard found by CLB and hope of gives you a runabout that will plane and you have pleasure from.

Will it plane? I have no idea, but CLB reckons it will! £4k for an outboard, £1k for a fabricated bracket. Test it. Works, great, tidy it up and do it all nicely. Doesn't - sell the outboard on eBay. Cost £1k and some effort.

And I appreciate all the help and suggestions they help with the decisions and planning. Noting is set in concrete yet as nothing has been changed and I could just put two inboards back In it and be done. ?

 

[thesaintlyone]

People are getting hung up on what the boat had originally. A typical 29ft sportsboat will get on the plane with 250hp as long as the torque is in the right place and the prop is well matched. It may not make 35 knots, like it did originally, but I suspect it would still achieve 25 without too much trouble. The two stroke motor I pointed to earlier would be better than a four stroke due to having better torque characteristics.

I have to say this was my thinking yes the original boat had two v6's producing close to 400hp but similar boats as you suggested havea combined 250hp Diesels power and they have no trouble as said nothing is set in stone for this project and the end of the day the aim is to have a usable boat that I like for reasonable money. not prove some it can be done kind of thing. If someone can point me in the direction of good value usable inboards for reasonable money it will definitely be considered. Perhaps the title should be Sundancer 290 Re-Power project. #notbloodysellingit ????

 

[BruceK]

People get tangled up in quoting hp. It's torque that you really want to get over the hump on to the plane which is why you can get a lower hp but higher torque diesel motor going head to head with larger hp petrol motors.

 

[Ferris]

http://www.marineenterprisesltd.co.uk/

Volvo D4-260 with 1000hrs for £7.5k. Get an outdrive off Coastal Rides?

 

[Grubble]

People get tangled up in quoting hp. It's torque that you really want to get over the hump on to the plane which is why you can get a lower hp but higher torque diesel motor going head to head with larger hp petrol motors.

Agreed. The Sealine S28 and Targa 28 both had twin 190hp V6s as the petrol option - there is a reason why bigger hp petrol engines are used compared to the diesels.

 

[CLB]

Agreed. The Sealine S28 and Targa 28 both had twin 190hp V6s as the petrol option - there is a reason why bigger hp petrol engines are used compared to the diesels.

You are ignoring the fact that back then there were no compact diesels with a higher output. 150's were use simply because the next option up was a heavy six cylinder.

 

[[2068]]

My S28 had KAD32’s which were 4-cylinder diesels, conservatively rated at 170hp.
It used to hit 34kts+ on flat water, lightly loaded.
Targa 30 with the same engines, maybe 38-39kts.

 

[CLB]

My S28 had KAD32’s which were 4-cylinder diesels, conservatively rated at 170hp.
It used to hit 34kts+ on flat water, lightly loaded.
Targa 30 with the same engines, maybe 38-39kts.

Yeah, but they came after the 150's. At the time of the s28 and targa 28 launch, the AD31 was the only Volvo diesel option available for these boats. That's why these boats came with 300hp in diesel form against the 400ish HP in petrol guise.

 

[petem]

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.

Lol!

 

[petem]

http://www.marineenterprisesltd.co.uk/

Volvo D4-260 with 1000hrs for £7.5k. Get an outdrive off Coastal Rides?

That would be perfect, and not a mongrel either.