Suggestions for a good technical UK boating forum

[Bullet-Proof_Biscuit]

AS above really, I'm looking to purchase a hull and DIY fit it out with my own engine & stern drive combination and will need answers to fairly complex technical questions on sizing and gearing & compatibility etc.

Could anyone suggest a good UK ideally South East regional forum for such things?

Thanks in advance.

NB. The boat should be something old and around 19' with a big engine and contra rotating props.

TIA!
HH

[Badda]

Check out the YBW forums.

[Hard-Drive]

YBW but do take some of the "advice" with a pinch of salt. There are some very knowledgeable people on there, and also a few idiots...

[Simpo Two]

Sounds like a fascinating project!

Why contra rotating? Are you worried about torque (as in a torpedo)? If you just want more 'thrudge' you could try what the early torpedo boats did and have multiple props per shaft! Or variable pitch?

Presume this is for offshore use? Fibreglass, wood or metal hull?

[Equus]

Bullet-Proof_Biscuit said:

Could anyone suggest a good UK ideally South East regional forum

Is there a particular reason you want such a local resource?

I'd have suggested the Boatdesign.net forums, but they're not UK-specific.

[pequod]

Simpo Two said:

Sounds like a fascinating project!

Why contra rotating? Are you worried about torque (as in a torpedo)? If you just want more 'thrudge' you could try what the early torpedo boats did and have multiple props per shaft! Or variable pitch?

Presume this is for offshore use? Fibreglass, wood or metal hull?

The technology has been around for some time!

https://www.youtube.com/watch?v=TdcWQpN8hl4&fr...

[Simpo Two]

pequod said:

The technology has been around for some time!

https://www.youtube.com/watch?v=TdcWQpN8hl4&fr...

Torpedoes have had contraprops for over 100 years; if they rolled it would bugger the depth keeping. But there are two things here - contraprop (ie two props counter rotating) and two props on the same shaft going in the same direction. Ultimately I suppose it's all about how much power you can transfer from engine to water with least cavitation and turbulence.

We look forward to the sea trials of 'BPB1'!

[pequod]

Simpo Two said:

pequod said:

The technology has been around for some time!

https://www.youtube.com/watch?v=TdcWQpN8hl4&fr...

Torpedoes have had contraprops for over 100 years; if they rolled it would bugger the depth keeping. But there are two things here - contraprop (ie two props counter rotating) and two props on the same shaft going in the same direction. Ultimately I suppose it's all about how much power you can transfer from engine to water with least cavitation and turbulence.

We look forward to the sea trials of 'BPB1'!

Indeed. I believe the offshore powerboat fraternity continue to use conventional props as they are mainly on 'ultimate' plane with just the prop in the water most of the time and power/weight ratio is probably a higher consideration and the complexity of duo props is an unnecessary complication?

Nice that we still have folk who wish to experiment with other options, though!!

[Bullet-Proof_Biscuit]

Ah right lovely thanks

[Simpo Two]

'Motorvator' might have some of the tecchy info you need. He races powerboats.

[Huntsman]

Ybw.

Facebook group Classic British Boats. Lots of knowledgeable people.

There's even someone on here that knows a little bit.

[Bullet-Proof_Biscuit]

Thanks for comments, joining all groups mentioned.

I'd like to refit an 80's power boat with something like one of these Volvo duo-prop drives because it would be more of a challenge, and I'm not a fan of boat anchor Mercruiser inline engines, excuse the pun:

https://www.ebay.co.uk/itm/Volvo-Penta-XDP-Sterndr...

I have however, also always wanted an Avon SR4 Searider from the 70s/80s. A refurb & refit of that might be just as fun and lots easier.
Transom modified for a pair of period 30hp outboards maybe. Can one outboard be made to counter-rotate though...? Thoughts bubbling..

[Huntsman]

I have an SR4. A pair of 30hp engines would be far to heavy and too wide. It really wouldn't work. I have a Honda BF40 which seems a really good match.

There are some contra outboard options.

[MOTORVATOR]

You'll probably get just as good advice here as anywhere.

Huntsman knows a thing or two and between us we can probably advise however if he suggests a T6 634 on a 280dp I shall probably counter with a 1650 on NXT as your best option.

Simpo has a great deal of experience as well if crawling up ditches is your sort of thing.

So come on then lets have some pics and we can help you spend some money.

[Simpo Two]

MOTORVATOR said:

You'll probably get just as good advice here as anywhere.

Huntsman knows a thing or two and between us we can probably advise however if he suggests a T6 634 on a 280dp I shall probably counter with a 1650 on NXT as your best option.

Simpo has a great deal of experience as well if crawling up ditches is your sort of thing.

So come on then lets have some pics and we can help you spend some money.

I always thought a marinised version of the twin turbo 2.7D from my old S-Type would be rather good. Not much bigger than my Yanmar but with 5x the power and 435 torques. Someone else can figure out how to get the power down

[pequod]

Simpo Two said:

MOTORVATOR said:

You'll probably get just as good advice here as anywhere.

Huntsman knows a thing or two and between us we can probably advise however if he suggests a T6 634 on a 280dp I shall probably counter with a 1650 on NXT as your best option.

Simpo has a great deal of experience as well if crawling up ditches is your sort of thing.

So come on then lets have some pics and we can help you spend some money.

I always thought a marinised version of the twin turbo 2.7D from my old S-Type would be rather good. Not much bigger than my Yanmar but with 5x the power and 435 torques. Someone else can figure out how to get the power down

Hydraulic drive to twin contra props maybe? There must be plenty of power in your old S-Type engine to at least experiment?

https://www.hydraulicmarinesystems.com/page/page/4...

I guess placing the engine weight f'ward may have an advantage in trim?

Edited by pequod on Thursday 24th September 18:08

[Simpo Two]

pequod said:

Hydraulic drive to twin contra props maybe? There must be plenty of power in your old S-Type engine to at least experiment?

I guess placing the engine weight f'ward may have an advantage in trim?

Interesting.

How about a second propeller behind the first, on an inner shaft spinning the same way but twice as fast - a sort of underwater afterburner

Call me Thornycroft lol.

[pequod]

Simpo Two said:

pequod said:

Hydraulic drive to twin contra props maybe? There must be plenty of power in your old S-Type engine to at least experiment?

I guess placing the engine weight f'ward may have an advantage in trim?

Interesting.

How about a second propeller behind the first, on an inner shaft spinning the same way but twice as fast - a sort of underwater afterburner

Call me Thornycroft lol.

I like your thinking!

Twin duo propped hydraulically driven outdrives with the inner shaft spinning up x2 for acceleration and then cycled back to sync, may be a winner?

[dvs_dave]

Bullet-Proof_Biscuit said:

Thanks for comments, joining all groups mentioned.

I'd like to refit an 80's power boat with something like one of these Volvo duo-prop drives because it would be more of a challenge, and I'm not a fan of boat anchor Mercruiser inline engines, excuse the pun:

https://www.ebay.co.uk/itm/Volvo-Penta-XDP-Sterndr...

I have however, also always wanted an Avon SR4 Searider from the 70s/80s. A refurb & refit of that might be just as fun and lots easier.
Transom modified for a pair of period 30hp outboards maybe. Can one outboard be made to counter-rotate though...? Thoughts bubbling..

Curious as to what’s so “bad” about Mercruisers? They offer the same types of product, but are generally cheaper to buy and service than Volvo’s. Are you wanting diesel or petrol power?

[gazzarose]

dvs_dave said:

Bullet-Proof_Biscuit said:

Thanks for comments, joining all groups mentioned.

I'd like to refit an 80's power boat with something like one of these Volvo duo-prop drives because it would be more of a challenge, and I'm not a fan of boat anchor Mercruiser inline engines, excuse the pun:

https://www.ebay.co.uk/itm/Volvo-Penta-XDP-Sterndr...

I have however, also always wanted an Avon SR4 Searider from the 70s/80s. A refurb & refit of that might be just as fun and lots easier.
Transom modified for a pair of period 30hp outboards maybe. Can one outboard be made to counter-rotate though...? Thoughts bubbling..

Curious as to what’s so “bad” about Mercruisers? They offer the same types of product, but are generally cheaper to buy and service than Volvo’s. Are you wanting diesel or petrol power?

If your only looking at ~19', then it's a toss up between V6 and V8, but depending on the Hull and weight, a V8 may be too much. A modern (last 15-20 years) will be knocking on the door of 50mph with a single prop behind a 220hp V6. If it was me I'd probably go single prop. It's a simpler lighter out drive (not much lighter, bit in a small boat every little helps), especially if your looking at that type of leg on ebay they are meant for the big block V8s and would be complete overkill for even a small block V8.

As far as choosing between Mercruiser and Volvo, there is very little to choose between one or the other. The majority of the petrol on the last 20 years for both brands were essentially the same rebradged GM engines. One thing that could swing it could be availability of parts and knowledge, Merc parts are easy to get, Volvo a bit trickier. The Merc Alpha One drive is also slightly lighter than a Volvo SP drive and will take a small block V8. A nice combo would be 220ish V6, Alpha drive and a High five prop. A V8 would have a bit more go, but also a bit heavier and potentially harder to find space for. Duoprop drives are more suited to bigger heavier boats that need and can make use of the extra grip. Think of it like putting 395 section tyres on the back of a Mx5, while in theory more grip is better, if the standard tyres aren't slipping, more tyre is just more drag and weight.

If you want to do something a bit different then the V8 from an LS400 is a popular swap in the US, but bear in mind lack of availability of parts may ruin a good weekend of holiday. Similicity and going a known route is always the best way to go with boats!