5hp v 6hp long shaft - difference

dylanwinter

dylanwinter

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www.keepturningleft.co.uk
I spent my day with the boys from PBO doing the outboard tests

the Mariner and Tohatsu came out best for my boat

they are both the same unit - both downrated 6hp engines

they both weigh 25kg

both had Katie L going at hull speed

so is there any advantage or drawback for going to the 6hp

will it be thirstier at a motoring speed of five knots

will the extra power be of any value on the Humber

help me lads

Dylan

https://www.youtube.com/watch?v=0afiWy2y-Uc
 
You cannot make it go through the water any faster than the max hull speed so from that point of view no advantage in the more powerful engine

BUT
Against a strong wind and a good chop I think on a boat that size you'll not want less than the 6HP!
Personally I might opt for an 8hp.
 
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It's your choice, but I once chartered a boat with a 2 blade folding prop on a Petter mini 6 i/b. It easily gave hull speed in calm weather & seas, but with a F6 on the nose + a knot or two of current we were making less than 1kt over the ground. What started out as a lovely days' sail, beating for the fun of it, ended up a looooong hard slog to get into shelter. SWMBO put the kids to bed & we just kept battering on into the dark, with no apparent progress (the mountains never seem to move!) for hour after hour.

I would always go for a few HP extra every time. The cost is seldom much more & you only really use the extra fuel when you open the throttle - and that's when you need it & are glad of the option. Waves & headwinds can really slow you down so it's nice to just open the throttle a little more if you need to get somewhere.

My 25footer had a 9.9 Johnson & most of the time it was half throttle, anything over just made her squat. But, given strong headwinds &/or overfalls I was happy to open the throttle & give it some welly, sure it used a lot more fuel, but it was well worth it.

You know how much of a cheapskate I am, so you can understand how important I rate extra engine power for emergencies. Some things, even I won't save pennies on! :D
 
I cant speak for the size of OBs that you mention.

I have a Tohatsu 9.8 and a Tohatsu 6 with identical props. The engines are virtually the same in every other aspect apart from the carb.

Both give very similar performance and economy for the same speed up to hull speed.
The only major difference is the 9.8 has more grunt if pushing through short chop. Where as when using the 6 I can expect to be stopped dead in the water by a succession of big waves.

Both engines will get the boat to hull speed the 9.8 will push it at about an extra half knot to a knot above hull speed but with a lot more noise and fuel consumption as the boat starts to get on the hump!

The 9.8 gives a bit more confidence in tough conditions.

I suspect you could easily uprate a 5hp to get the benefits of the 6;)

Recent experience with a Johnson 5hp was that a change of the main jet in the carb realised 6hp!
 
I had a 4hp Suzuki 4hp on my Anderson 22 and thought it was plenty. The Anderson is an easy driven hull shape with not much windage either.

I discovered the only difference between the 4 and the 6 hp Suzuki was a little metal plate that resztricted the throttle movement was bigger on the 4. Was tempted to order the plate for a 6 as a spare part to convert to 6 but never bothered in the end,

If theres no weight difference I would go for the extra hp unless its a lot more money.It will only be thirstier if you keep the throttle wide open. Most boats only need about 2/3d throttle unless against strong winds.
 
TSB240 said:
I suspect you could easily uprate a 5hp to get the benefits of the 6;)

Recent experience with a Johnson 5hp was that a change of the main jet in the carb realised 6hp!
Click to expand...
It's a carb change on the Tohatsu. About £240 last time I looked.
Cheaper to get the 6HP to start with. All the power will be at the top end so in normal use it should be just as quiet and economical as the 5hp
 
I suspect you could easily uprate a 5hp to get the benefits of the 6

Recent experience with a Johnson 5hp was that a change of the main jet in the carb realised 6hp!
Click to expand...

Sometimes you can, sometimes you cannot. In some cases its as simple as removing a stop that prevents the throttle valve opening fully
In others a carb jet change does the trick

But I have an Evinrude 6 hp.. On the face of it the same engine as the 8hp with a different main jet but further investigation revealed that the inlet manifold and crankcase are different giving completely different porting. So not a viable proposition to up-rate it after all.
 
VicS said:
You cannot make it go through the water any faster than the max hull speed so from that point of view no advantage in the more powerful engine

BUT
Against a strong wind and a good chop I think on a boat that size you'll not want less than the 6HP!
Personally I might opt for an 8hp.
Click to expand...

+1
The extra horse power is needed to power through the waves and tides rather than to increase the speed. To move the boat, at normal conditions, at sub hull speed, the fuel consumption will be practically the same whether a 5 hp or 10hp.
 
I had a Mariner 6hp four stroke on my Hunter Horizon 23 and it was just right. I wouldn't have wanted any less and didn't need any more. At 3/4 plus throttle the boat was starting to squat down by the stern and not going any faster - but I could punch into head seas and tow a dinghy and occasionally other boats so I appreciated the extra power. Your boat is of a similar size. I would go for the 6hp.
 
I've been following this thread with interest, as I have a similar problem. However, can anyone tell me how to calculate 'hull speed'? I've googled it and punched in some figures into a calculator. The result is about 6 knots. I can get 7-9 our of her with a favourable wind and sea state without the engine. So surely the my max hull speed must be more. Now I'm really confused!!!
 
Hi


if reliability is a factor then i would go for a yam or honda 4 stroke 5hp than a bigger other make. Oh and before everyone screams because I am a dealer I would say this, please do the maths.

look at the price of a 2nd hand honda around 5.5 years old i.e. still under warranty and you will find that that engine has only devalued at around £50.00 per year. Thats not bad for saving your life and its even better when you take £20.00 of that as a guide to devaluation of 2nd hand 2006 engine per year. Even if you doubled the devaluation of the honda and halved the 2nd hand engine then that is still only £90.00 per year so is this really a lot?

yes extra power is always good, but only if it works.

Good luck and safe boating
 
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