Approx max design spped. Help needed.

[drewstwos]

Hi everyone,
My boat is a Mitchel 8 metre. (26ft) beam 10 ft and is a semi displacement hull. The LWL is 24 ft. At the moment it has a 50 hp Perkins inboard diesel, which is OK,, but SLOW. like 6 kts at 2600 rpm. It also has, an aux a 15 hp OB, hung on a strengthened swim platform. When run together it adds about 2 kts, (I did post a little bit about doing this as an experiment some time ago).
My questions are,,,
1..,Any ideas as to what the max design speed might be for this type of hull.
2..What hp outbosard would give me this. in conjunction with the diesel.

I have been thinking about putting a bigger diesel into her, but the cost is totally prohibitive (for me). and thinking that the red diesel derogation may well be a dead duck soon, the cost of fuel for either type may well be nearly the same.
I was told that a 100 hp inboard diesel might do the trick, but I doubt it.
I would dearly like to be able to do 15 kts... as this would cut timewise my planned next year trips by nearly two thirds,, (a weather consideration in the Irish sea)
It would also get me to the best fsihing spots a darn sight quicker. Its a tad frustrating being passed by all the others
So. any thoughts on this wouldbe much appreciated.

Thanks in advnace

Drew.

 

[longjohnsilver]

Should be the square root of the waterline length x 1.25 or there abouts.

Or to put it simply for your boat will be about 6.2kts.

I'd think that to get it well past displacement speed you'll need at least 150hp, anything much less than this and I think you'll be wasting your time (and money!)

 

[jcmmarine]

Years ago had a 27 with a 50hp, would be surprised if you got more than the 7.5 knots I used to achieve.
You may get a slight improvement by changing the the prop. Check with one of the Prop Calculation programmes available on the internet.
Unless you can get a bargain second hand unit it would praobably be cheaper to change the boat than fit a new engine that would give you 15 knots cruising speed.
I ooccasionaly go out in a friends Mitchel 31 Sea Angler with a 115hp 6354 Perkins. On a good day, with a clean bottom, calm sea and the wind behind, it can just about make 11 knots, but with a great deal of noise, wake and fuel.

 

[ColinHuggins]

Re: SNAP!!

So you have a Mitchel 8 metre (26ft.).

I have a Newhaven SeaWarrior 27, designed by Alan Hill and actually 8.4m (28ft) LOA. A modified version (longer wheelhouse) can be found as the Mitchell 28.

If yours is the same it starts to plane at about 8 knots and should attain 15 knots with a 120HP inboard according to the Designer. Mine is driven by a 1968 Perkins 6.354 and consistently does 12.4 knots clean and dry over a measured distance: that's four runs with a stopwatch, none of your gps fantasy! BUT the bilge keels are oversize and I think she's overpropped too - the jury's still out on that one.

Are you talking about the Newhaven yard who produced the Mitchell 31, or the Parkstone yard with a similar name who also built boats?

Suggest you PM me and exchange info.

 

[ColinHuggins]

Re: Perkins 6.354

Oh, you can get a secondhand 6.354 for £1000 - if you don't mind pulling a tractor or combine harvester to bits. Chase them down under the name of Massey-Fergusson . . .

 

[Kawasaki]

Re: SNAP!!

Got a bit of info for you.
Friend of mine (nice guy but garbage at driving boats!) has owned 2 Newhaven Sea Warriors.
Because of His lack of desire to drive Her moor Her or plan anything! I had the pleasure of driving His craft quite often.
The best one He owned was a late 80,s build with a Ford Sabre 150 Turbo mill.
She attained 17knts and cruised effortlessly at 14/15.
Very nice boat called "Moryn Warrior"
No. 2 was an earlier boat (Don,t know why He bought Her but that,s Terry!)
Early 80,s fitted with a Ford Sabre again but a 120 hp normally aspirated.
Had overheating probs when He bought Her but by the time I had sorted it out for Him He decided to sell Her! This one was Re named by My Oppo "Mon Warrior"
Re naming ceremony was not carried out in the correct fashion in My opinion, cos on the Delivery Trip to Dublin (He eventually sold Her!) She conked out 10 miles off!! (That,s another storey must post it soon it,s a laugh!)
Anyway for your edification! The 120 Sabre was flat out at 14knts and criused at 11 at 2250 rpm

Nice seaworthy boats but one can be a bit busy on the Helm when the Oggin is up your arse! /forums/images/graemlins/grin.gif

 

[ColinHuggins]

Re: NOT a Mitchell after all

Having seen pic we are barking up the wrong tree - new posters please note that any resemblance to a Sea Warrior 27 aka Mitchell 28 is fortuitous.

Curioser and curioser sed Alice . .