Confused, you bet'cha! Please sort this one out. Help!!

[drewstwos]

Confused, you bet\'cha! Please sort this one out. Help!!

HI all

At the end of last summer I decided to re engine the boat, as the little 50 hp was just not man enough to cope effectively with the tides and currents we get here on the Isle of Man. I have now installed a 160 hp turbo beast which is now lined up and bolted very firmly down.
So the next thing to do is sort out the prop, and here’s the conundrum.
I contacted two very well known firms, both recommended by forumites who had had excellent service.
I was baffled before, but now I am totally confused. Why?
Well. The old engine had a 19 by 11 prop, turning at 3000 rpm giving a mighty 6.5 kts. Boat a 27ft Aquastar, beam 10ft LWL 25.5 ft weight 4.5 tons draft 3.25 ft gearbox ratio 2:1.
But…and here’s the rub
New engine turns at max 4250 rpm 2.13:1 gearbox
One set of figures for the new engine ..a 20 by 14, and the other 18 by 12. So which is right? You can see now why I am totally confused. One over the original and one under.both suggesting a tad under 16 kts
Being a poor old pensioner, getting it wrong at about £500 a pop (prop..excuse the pun) would be painful to say the least.

So, my most wise and experienced fellow forumites, your thoughts and conclusions would really be much appreciated.
I look forward to some very interesting replies.
Over to you……………

Cheers /forums/images/graemlins/mad.gif
Drew.

 

[FullCircle]

Re: Confused, you bet\'cha! Please sort this one out. Help!!

When I repropped my raggie, Kiwiprop pointed me to this calculator. Seemed to work for me, or at least put you in the right ballpark.

Merry Xmas

Jim

 

[pheran]

Re: Confused, you bet\'cha! Please sort this one out. Help!!

If you Google 'propellor calculations' you can get access to numerous downloadable software packages. Try a few of these and see if a consensus emerges.

 

[rich]

Re: Confused, you bet\'cha! Please sort this one out. Help!!

Try contacting Aqua star, I am sure they can tell you.

 

[kippers26]

Re: Confused, you bet\'cha! Please sort this one out. Help!!

why not try using your existing prop, and if necessary having the pitch adjusted to suit, it will be cheaper than a new prop.

 

[brianrunyard]

Re: Confused, you bet\'cha! Please sort this one out. Help!!

Using your figures in Castlemarine's PropCalc gives 18" x 13" and 17.6 kts

 

[volvopaul]

Re: Confused, you bet\'cha! Please sort this one out. Help!!

What speed are you expecting, I doubt it will plane as its not that sort of hull, I would contact geoff wilson at aquastar, i would also look at using your existing prop as the diameter sounds right, yout best fitting the biggest diameter prop as possible rather than a big pitch thats the golden rule, have you also fitted a new shaft thats capeable of the new hp as it will shear off at the tapered ends if not correct size/ material, 11 inch pitch is small and may not be able to repitch larger due to the size of the blade where they meet the hub as more stress on a repitched blade may not have enough area of contact on the hub and break off.

You may be able to repitch about 3 inch max before you get trouble.

 

[drewstwos]

Re: Confused, you bet\'cha! Please sort this one out. Help!!

HI

First many thanks for the advice, It's much appreciated.

I also used castlemarine propcalc. and would think that they are more right than the other.
The shaft diameter is 1.5 inches and I have been assured this is more than enough for the increased HP.
My hopes are to be able to cruise at about 12/14 kts max at 16. as this will take care of the adverse tides and currents hereabouts.
The hull design is for a max of about 18 kts with even more power, so this is the best I can do.

Cheeers and a Merry Xmas to all

Drew.