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I think you will need somewhere around 275-300hp to get reasonable planing performance.
Or 50hp to plod along at displacement speeds.
Or 50hp to plod along at displacement speeds.
1992 Sea Ray Sundancer 290 Outboard Conversion Project
- Thread starter thesaintlyone
- Start date
thesaintlyone
Well-Known Member
What is planing why do I want it where in the UK can I actually use it???? If my greatest use of this boat is driving up the Medway into the Thames estuary and up the Thames then am I really gonna make the most of high speed planing. Looking at the prices of used outboards those in the 175hp - 200hp category appear to be a nice price range for starters.
Seastoke
Well-Known Member
Didn’t Shetland boats have a bolt on bracket for out boards on their 4+2
[2068]
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The thing to consider is that that to get planing at all, you need to get “over the hump”.
(past the transition from hull speed).
At this point, you need all the thrust you can muster, else it just won’t get there.
I’m unsure whether 175-200hp will achieve that on a 3500kg boat, or just get the nose in the air at 12kts pushing a large wake.
If you’re okay with 8kts, then a 50-75hp should be enough.
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LBRodders
Well-Known Member
^^ sorry having a nightmare posting.
175-200 horses is a white elephant...
Too powerful to trundle around and not enough for any performance or planing, you might as well save money and buy a 50hp..
I think you need to start with working out what you actually want. What are the requirements and budget etc...
175-200 horses is a white elephant...
Too powerful to trundle around and not enough for any performance or planing, you might as well save money and buy a 50hp..
I think you need to start with working out what you actually want. What are the requirements and budget etc...
MrB
Well-Known Member
Would a 50hp engine actually move it? I had one on my old Fletcher but this boat is close on 10m long? Looks like it had 380hp originally. To me, with some wind and tide that thing with a 50 would be going backwards?
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Hopefully not - a V6 200hp petrol at 3000rpm might only be acutally delivering something like 50hp.
Even if it’s 75-100hp, it will still probably need a new propellor with a much smaller pitch than you’d usually fit to a planing boat.
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thesaintlyone
Well-Known Member
Let's not forget these boats were designed and built in the states. Fuel is cheaper and spaces are bigger. Twin V6's or 7.4L mercruisers work over there but what would be powering a similar sized european boat????
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Twin V6’s or a 7.4L.
Or diesels with slighly less power but more torque, e.g. twin AD31’s or KAD32’s (150hp or 170hp each).
julians
Well-Known Member
Euro boat would be similar power level, might be diesel,but power will be similar.
You need X horsepower to achieve y speed for a given boat.doesnt matter whether it's American boat or euro boat.
Having said that , nearly all marine petrol inboards are based on the same GM block (sometimes ford).
simonfraser
Well-Known Member
my mobo is 3500 kg, has one 275hp inboard diesel, agree can't see 200hp petrol with ? less torque being sufficient to plane.
julians
Well-Known Member
This might be useful for a very rough guide, I have no idea of it's accuracy, but it looks worth a read, and might be helpful.
https://betamarine.co.uk/engine-selection-for-yachts/
It seems to assume a displacement hull,but I'd guess it'll be good enough for a rule of thumb for a planing hull at displacement speeds.
https://betamarine.co.uk/engine-selection-for-yachts/
It seems to assume a displacement hull,but I'd guess it'll be good enough for a rule of thumb for a planing hull at displacement speeds.
MrB
Well-Known Member
It wouldn't be a 50hp outboard that's for sure! I do trust FP's input into this and have admiration for you trying to power a nice boat cheaply but it's going to look daft with a 50hp outboard on the back! (and I still don't think it would work)
I used to live in Florida and had 6.6l and 5.0l cars, they weren't particularly good either but i get why they were so big.
*Added to say I'm not knocking the idea what so ever, this will be an interesting thread and I hope the outcome will be great .
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thesaintlyone
Well-Known Member
Don't worry I have no interest in putting a single 50hp on the back. Just trying as others are I'm sure trying to establish parameters. Sand I'm not knocking American design just stating that you have more room to play over there than we do ?
thesaintlyone
Well-Known Member
OKi let's assume for one minute I decide to go with two outboards in the 120-150hp range how does that sound????
[2068]
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A couple of 150’s sounds like you’d be in the zone for planing performance.
LBRodders
Well-Known Member
Mechanically fine....But where are you getting these from? A matching pair will be difficult to find used, if not impossible. You may have to buy a boat with them.
A new pair will cost a more than a suitable inboard.
The difference in project cost between river plodding and sea cruising is night and day...There's no cheap in between'ie
jrudge
Well-Known Member
Interesting. A search of eBay shows one 150hp outboard for £8k.
Nothing on completed listings. So definatly rare. New ones £13k each
I suppose high power outboards are relatively new compared to say a 50 that has been around for decades ?
Nothing on completed listings. So definatly rare. New ones £13k each
I suppose high power outboards are relatively new compared to say a 50 that has been around for decades ?
Grubble
Well-Known Member
Have you researched what these boats were fitted with originally? It looks like the 4.3 carburettor Mercruiser was the minimum spec and they produce 190 HP. If you put smaller engines on it, you don't have any guarantee the boat will plane, especially when the boat is full of junk and the bottom is covered in river slime and weed.
CLB
Well-Known Member
A typical 29 foot boat will plane with twin diesels delivering a total 300hp. In single diesel engine format, it should plane, just, down to about 200hp give or take some parameters. Petrol engines do not deliver the same torque characteristics, so you may need more HP. Twin 120's are probably not going to work, although a single 250 might.
