What size outboard?

[crackoff]

Hi all,
Trying to get a rough budget needed to get my boat back in the water, where it should be. But, even though I have managed to work out a rough budget for the work needed, (GRP, re-fit, flooring etc) I have absolutely no idea the size of outboard I'm going to need. Can you help?
First and foremost, its 16 ft long and 6 ft 6 wide (see photos) and is going to be used on the Thames and maybe for a trip around the Broads every so often. Obviously the flow of the Thames will mean a larger outboard, and would prefer to get a motor that can cope easily rather than one that I'll have to rev high to keep a sensible speed, but where do I start?
ps... The transom is 21 inches, so I take it I'll need a short shaft?

 

[timtap]

Probably for the application you require I would probably use a 40hp with a standard shaft length

 

[cliff]

I would have thought 25 would be enough considering speed limits etc on the Thames and the Broads unless you want to use her for water skiiing in which case 40~60hp
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Artificial Intelligence is no match for Natural Stupidity

 

[VicS]

Firstly you will need a long shaft. Standard (or short) shafts are 15", long shafts are 20" The cavitation plate should be just below the bottom of the transom. (3/4 to 1")

You don't say whether you mean the tidal Thames or the non tidal Thames. If you mean the non tidal Thames and the Broads then speed limits apply so you will not need anything like the power being talked about. 8 or 10 will be more than adequate. 6 hp pushes my 19 ft around quite happily.

However if you want to get up to planning speed then yes you will want a good bit more and if you want to get an adult water skier up then you will need 40 or more I would guess .

A question on the "Non repair outboard discussions" board of the iBoats Boating forums could well get you some very useful advice but say exactly what you want to achieve.

 

[Lakesailor]

Like Vic says the shaft lengths are shown here http://www.smalloutboards.com/shaft.htm

Do you have steering and remotes? If not look for a motor being sold with them as the cost of fitting them afterwards can be very high. The cable fitting kit for motors can be £170 alone.

If you can stretch to a four stroke that would work better for slow speed rivers and tidal Thames as it would run better at low revs.
I'd have thought something in the 15-25HP range would have been best. I had a 70hp on a 15 ft boat on Windermere and whilst it travelled very well at 6mph (the lower limit) the excess power meant that the prop speed to achieve 10 mph (the higher limit) was delivering too much HP and it would climb on the plane and rush off to over 20mph. (that's my excuse).
So unless you want to fly around at 35 knots I'd limit your engine to under 30HP certainly.

 

[crackoff]

Yes, this is where it has been confusing. Although through asking around, I had come up with 15- 20 hp, (just through picking the average of what I was being told) I thought I'd better get more feedback, But its the shaft length that I'm getting so confused over. So far, its a 50/50 split as to whether I should have a short/standard or a long shaft, though the link provided by lake sailor should stop my worrying...
Thanks all, Martin

 

[moody1]

[ QUOTE ]
Yes, this is where it has been confusing. Although through asking around, I had come up with 15- 20 hp, (just through picking the average of what I was being told) I thought I'd better get more feedback, But its the shaft length that I'm getting so confused over. So far, its a 50/50 split as to whether I should have a short/standard or a long shaft, though the link provided by lake sailor should stop my worrying...
Thanks all, Martin

[/ QUOTE ]

It's normally accepted that there are only two shaft lengths, short and long. But in fact some engines have 3 shaft lengths and even extenders. But for your boat you would not go wrong with long shaft. If yoyu buy short shaft and it's too short you are up a gum tree. Buy long and it's too long, it will still work. Being deeper by that few inches will despite experts saying its not good, do no harm to your speed or use.
As to size I would agree withy the 25HP for general use, and bigger if you want to do water-sports. One point that no-one seems to have made is the 2st vs 4st. Personally a 2st 25 - 30HP would be our choice, with oil injection if possible. This would have to be second-hand of course. Why ? Usually lighter and more compact. Plus of course considerably cheaper.

 

[Lakesailor]

[ QUOTE ]
One point that no-one seems to have made is the 2st vs 4st.

[/ QUOTE ]
Oh. You didn't read my post then?

 

[VicS]

[ QUOTE ]
its the shaft length that I'm getting so confused over

[/ QUOTE ] You might like to take look at this topic regarding shaft length. F_R and Tashasdaddy are two people who really do know what they are talking about! click me

As far as 2 stroke vs 4 strokes are concerned LS is probably right about the 4 being happier than a 2 about running at low revs.

Acceleration is were 2s score over 4s, I understand. Although a 2 and a 4 of equal power will stay neck and neck with each other at speed, from a standing start the 2 will beat a 4 easily and the 4 will never catch up.

Going back to the size. Really you need two engines. A large one for high speed use and water skiing etc and small one for low speed use. Thats why you often see them side by side on the back of the same boat.

 

[crackoff]

Not really bothered about going fast, nor am I likely to wish to get a skier up... Far too lazy for all that )

 

[crackoff]

Cheers for the link Vic...
Just seemed to add to my probs...
First reply... 90hp??? /forums/images/graemlins/laugh.gif
Though to be fair, the reply came from a nice American fellow
and they do like thier big engines...
You seen the way they fish for bass? Proper sneak up on them with a V8 powered speed boat at 70mph...

 

[Drascomber]

I don't understand all this talk of 30hp plus engines.

You talk of the flow of the Thames. Well, except in winter you are unlikely to experience more than 1.5 knots of flow on the non tidal Thames where 2.5hp would be adequate - as it would on the Broads. However, 5hp to 6hp will get you a remote tank and is more likely to accomodate remote steering, gears and throttle. If you want electric start they tend to start at 8hp.

If you are likely to visit the tidal Thames where a flow of 4knots is common in some places I suggest you think in terms of 5hp and upwards with 8 or 10hp for more in reserve.

Unless you want planing speeds or enjoy spending ludicrous amounts of money on petrol - particularly on older two strokes - forget the gas guzzlers.

 

[VicS]

[ QUOTE ]
I don't understand all this talk of 30hp plus engines .................... 2.5hp would be adequate

[/ QUOTE ]

Its a Power Boat not a raggie, He wants to pull the chicks. The more HP the better the chicks.

What he really needs is twin 200hp with long legs :-

 

[Drascomber]

Yeah but even on tickover those motors would get you there far too soon. In certain circumstances there are definite advantages to travelling slowly. Still I suppose they could always run out of petrol . .