Going Coastal in a Shetland 4+2

[QBhoy]

I have no expectation of planing and trying on a canal (The Wey Navigation) is naughty to say the least! May well open up on a quiet stretch of the Thames.
The boat is 8m overall, 21ft plus engine and frame. 1100kg plus 90kg engine, water, fuel and 2 min adults another 200+kg. It feels stable, not as light as a 535!

Clive I don’t think there is an issue with the engine. No doubt the boat is designed to plane ideally…but I’d imagine even with 35hp and the right prop…you’ll be able to run as quick as any other displacement boat, in terms of speed, should you need to.

 

[ChromeDome]

If 1500 kg total and everything is ok (engine, clean hull, prop) 35 hp should be able to push it to 17-19 knots.

The challenge would getting it up on plane, might take some weight distribution and trim. At 17 kn it should plane, pretty stable once out of the water.

 

[gordmac]

Neither of the 535s I had would plane three up with 50hp.

 

[mabbs]

Neither of the 535s I had would plane three up with 50hp.

That’s surprising, our old 535 with a yam 55 would always get on it, that was with 2 adults, and three kids and a Labrador and 75l petrol, secret was for the old man to send two of us up into the bow. We had a 35hp one year when the main engine had a problem, now that would barely plane with two on board. Happy days!

 

[Clive-D]

Thanks for that QBhoy, you’ve raised another good point. I have a low speed prop at the moment and will need to change it, it’s 11.4x12HQ, any recommendations for a Honda BF35A appreciated, understand the principles but don’t want to buy the wrong one.
ChromeDome, that would be nice. Changed the old 150L water bag for a 50L last year. Think now it might have been there for weight distribution. I’ve got a trim on the throttle so be good to try.

 

[ChromeDome]

My guesstimate is based on a prop calculation. To be more precise and suggest optimal pitch, I'd need to know the rpm you reach at WOT (wide open throttle) but I assume that is unknown until you do the tests.

When factoring in that your gear ratio is 2.08 and the recommended WOT range is 4600-5600 rpm, one aims for the 5600 to get the full 35 hp (all as per the manual). At 5600 the prop shaft spins at 2692 rpm which with a 12" pitch gives 26,59 knots (not taking prop slip into consideration).

This is obviously not true if the hp enables 17 kn as a max. - that actually would mean the rpm is held at only 4000 (ish) and the engine would be lugging/overloaded. Alas less weight and/or lower pitch comes into play.

Since a given number of horses can push a given weight to a certain speed, the by far easiest and cheapest way to go faster is to remove unneeded weight.
Empty water tank, remove as much as possible at least for the tests.

Edit: Bear in mind that boats tend to put on weight over time, so the actual GW is more reliable than original manufacturer specs.

All IMO...

 

[Mark26]

You might enjoy watching this, he makes all the rookie mistakes, like straying into an active firing range and had obviously never seen a chart when he asks “how is one supposed to know it’s there?”
Very likeable chap though, and he even goes out to the scilly isles.

 

[Mark26]

My steel built cruiser which is based on the Thames, only cruises at 6/7 knots.
When coastal, I simply go with the tides, there’s no point in punching them.
6/7 hours of motor cruising is enough anyway. Then I’m glad to turn it off and have some peace.

 

[Clive-D]

You might enjoy watching this, he makes all the rookie mistakes

Looks good, I’ll have a watch. We binge watched Sailing Uma in lockdown. They bought a boat for $3500 did it up in Miami, crossed the Atlantic, gone as far north as it’s possible in Norway and are south in Iceland. They ditched the engine and are 100% electric. Interesting journey, more adventurous than my plans!

 

[Seastoke]

We took a 4/2 down to the med had a great time , but had an eighty on the back.