Submitted by graham.kill on
Intrigued by the Macgregor/Tattoo website video of motoring at 20kts, the other day with only two adults on board and on calm water I: dumped the water ballast; lifted the keel; lifted the rudders; and trimmed appropriately as speed developed. Opening up I could only get to about 3100-3200 RPM and 11 kts (accounting for tidal flow). Am I missing a trick to develop more speed?
7 Comments
Re: Top boat speed
Submitted by gerard.cairney on
3100rpm sounds wrong flat out, check your prop/engine configuration.
For 20knts you would have to strip everything out of the boat and attach a helium balloon to yourself.
New website says up to 20mph (17.3knts) http://tattooyachts.com/under-power/
Realistically I can get maximum 15knts flat out @5100rpm with a couple of people on board and all my accumulated junk.
Better to throttle back a little to save fuel and go about 12knts. That's still twice as fast a most sail boats.
Fastest I have recorded SOG was 19knts with an 8knt tide running with me. I have a 26M with 60HP High Thrust Yamaha, great machine!
Gerard ~ Oran na Mara ~ M26
Re: Top boat speed
Submitted by rick.jones on
I agree with Gerard, those claimed figures can only be achieved with a completely stripped out boat and one (small?) person on board. MacGregor also always use MPH rather than knots (15% difference).
However, your lack of revs is because your engine is over propped. The correct prop will allow the engine to reach peak revs, and increase the top speed. The standard prop on most 50hp outboards is appropriate for something like a 15' RIB, typically 13" pitch. A Mac 26 needs more like 11", or even 10".
Many people don't realise when buying the engine that they need to specify a prop appropriate to the boat. A shallower prop will also make manoeuvring easier, as the boat will go slower at tickover revs.
Rick Jones (Treasurer), former 26X & 19 owner, Isle of Wight
Re: Top boat speed
Submitted by graham.kill on
Dear Rick and Gerard, Great feedback. Many thanks. Graham
Re: Top boat speed
Submitted by dennis.bolt on
I have a Yamaha 50hp which came with the boat new and so I assumed it was fitted with the correct propellor. Max revs. I can get is 4400, which I felt was a bit low. Best speed I've had with ballast out but with some weight in the boat is 14.5kts
Re: Top boat speed
Submitted by rick.jones on
Never assume the prop is correct! Engines are sourced and fitted by dealers after import, not by MacGregor, and can you be sure they took the trouble to specify the prop when ordering the engine? If like most people you only motor at modest speeds and don't care about performance you'll never realise if the prop is wrong!
Rick Jones (Treasurer), former 26X & 19 owner, Isle of Wight
Re: Top boat speed
Submitted by simon.armitage on
I covered 11 1/2 miles in 34 minutes on the river Thames last year in my 2005 26M with a factory fitted Yamaha 50 hp 4 stroke (500 hours).
Haven't managed to repeat this though as have since moved to the Blackwater and have not experienced similar conditions yet.
My crew weighed in at 18 stone+, I am just over 12, the mast was lowered and the usual bedding, sails,rigging, junk, water etc filled the cabin and with around 40 litres of fuel (at least when we started)
I think it was a fluke, slack water, dead calm, no wind, surface like burnished copper, empty ballast so full throttle with my crew trimming as we lifted, then we reached the Woolwich Ferry and had to slow down. Didn't have enough fuel left to repeat the adventure so sadly returned to port where our Marina owner told us we were 'flying' and looked very impressive. Sadly, he did not have his camera with him so no pictures.
It was quite exhilarating but I don't expect to do this too often as circumstances have to be right.
I also managed to record 10 knots (tide assisted) on a broad reach while circumnavigating the Isle of Wight this year, but I don't have myself down as a speed merchant.
Keep trying and experimenting.
Rick's advice on props is well worth investigating.
Simon Armitage
Sowenna (26M)
Re: Top boat speed
Submitted by roly.simpson on
I recall that I managed 18 knots on flat water without ballast when my Mac M was brand new 10 years ago. Now with kit on board and a slightly dirty bottom I am lucky to get 13 kn.
The agent recalled my prop soon after I bought it saying that it was the wrong one but if anything I thought the original was better. I cant recall what the original was but I think I currently use a 13 K. Seems to get to 53 revs ok.
Roly