Shetland 536 engine sizes

[Gabe]

I bought a 536 last year with a 75 hp 2stroke Yamaha of uncertain age.
I have an almost new(10hours only) 30hp Tohatsu off my old dory.
I know the boat is rated at 60hp but what are folks ideas about running with only a 30hp.
I.e. will it plane? Likely top speed?

Comments gladly received
Gabe

[dellquay13]

The 30hp is just about as wrong as you can get. It's too small for the boat to plane, and too big for efficient displacement speed.

I have had various outboards on my F4 (same hull as the 536) over the years. It came with a 40hp 2str which would only just plane in a flat calm and full throttle. I changed to a brand new 50hp 2str which was better for planing but still working in the top 80% throttle all the time, and I was almost pouring petrol in from a bucket. The 70hp 4str is loads better with power in reserve, but not as economical as i had hoped and without the inherent hole shot performance of a 2str. (it's the same engine as the 50hp 4str model but tuned up and higher revving to give 70hp. The lightest 70hp around but it also turned out to be the thirstiest).

A 30hp 4str won't get you on the plane, but will hit displacement speed at fairly low revs. Anything above about 30% throttle will just increase the size of the bow wave you make without much increase in speed, meaning you are working in the first bit of throttle lever range only, very sensitive to tiny adjustments.
The max displacement speed for the hull will be about 5-6 knots, maybe 7kn with loads of revs and a massive bow wave in front. With enough power to defy gravity and climb the bow wave you will start to plane at about 12kn getting more stable at about 14kn and above. The hull has a comfortable cruising speed of 16-18kn. The top end speed is engine dependant.
Ideal engines would be either 60hp+, or 6-10hp.

The hull is rated to up to 70hp, but in the days when it was built, they were all 2str and a fair bit lighter than a 4str. I don't think the power needs reducing, but the boat balance might need adjusting.

Your 75hp 2str will drink about 1-1.5L per mile at cruising speed I reckon. My 70hp 4str is more like 0.6-0.7L. My old 50hp 2str was 25mile to a 25litre fuel tank, now i get about 40mile from the same tank.

[dellquay13]

(I still miss my old Dory, it was like boating in a rocket powered shoe box lid, very wet but great fun in a flat calm under the Greek sun...)

[Gabe]

Delquay 13,
A very big thank you for your informative response to my question. I already have a 60hp Tohatsu on order but having seen several Shetland 535/536 for sale with small engines and claims to be adequate, I was just wondering

Gabe

[dellquay13]

The 535 and 536/F4 are very different hulls, the 535 was a simple shallow v, and about 30% lighter than the gull wing 536/F4, with it's foam filled hull, expanded interior fittings and gull wing shape. The 535 can get away with engines around 35-40hp if the interior has been stripped back bare for fishing.

[Gabe]

Some good news: Burry Port Marine Services have informed me that my new Tohatsu 60 will be here by the end of this coming week. Been on order since October.

These guys are great if you are in South West Wales. First class unbiased advice.

[Adam]

I have F4 with 4 stroke 70hp Johnson. A bit heavy lump but the boat goes pretty nice and amnvery happy with my setup.