GEARBOXES

[Micky]

Could a boat fitted with two direct drive gearboxes be changed to a pair of 2 to 1 boxes, and would there be any advantage in doing it, if it would be viable what else would need changing, EG props or shafts etc.

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[scottie]

yes but why ?

all propeller sizes are a compromise so get propeller calculation done showing the results of various options

if the engines are original then they should be matched to the correct red gear

increasing the reduction rartio increases the torque which requires larger stern gear and propellers etc may also require the engine beds to be raised if a drop reduction is used

it may not even be posible to get the larger props in the space etc


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[Nauti Fox]

Have a chat with Mike at Lancing Marine,funnily enough I'm going down that route at the moment (incl engine change)
Seems to be very knowledgable and a pleasant bloke!
Regards, Al.

<hr width=100% size=1>No dear,the water goes in the other one.

 

[ccscott49]

Cant see any advantage for your type of boat, just an expensive pointless exercise. IMHO

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[Divemaster1]

< I'm going down that route at the moment (incl engine change) >

So Al, .... what does it become ??? Same make, but less hours or are you changing for something of a different make/rating ??


<hr width=100% size=1>Alf

 

[Nauti Fox]

Hi Alf,as everone suggested,new engines or rebuilt,not ancient GMC's.Considering lower horsepower than originals,but on a 2:1 gearbox as long as I've got room for the props, got to check the shafts are ok and get some accurate measurements of the space available.
I'm hoping that with the weight loss and USABLE horsepower it'll be able to get to 16-17 knots.
Al.

<hr width=100% size=1>No dear,the water goes in the other one.

 

[Divemaster1]

Great stuff Al...

Not sure how much a weight loss will do for you in terms of speed actually ... the boat is built like a brickhouse (underwater at least), so you carry a lot of laid up weight. Considering that you are thinking about living on the boat you need to know a couple of things about Kjetil's (my Norwegian friend's) boat.
He is running with the "lighter", straight six, 300 HP Cummins ... but the boat now have a slightly lighter stern, lifting the side edge of the stern hull sides above water, which generates sound as ripples of water hits the boat from the stern and side (not always peaceful). He has a 11 foot RIB with a 15 HP outboard in the davits which improves matters a bit when still.
Also boat gains speed as bow goes down (presenting a longer waterline length), but gets to be "wet". Also when you are below there, you could consider extending the rudders as my friend have, as the original size ones are not very effective at slower speeds...

<hr width=100% size=1>Alf

 

[Nauti Fox]

The GMC's are about a ton each with gearboxes!I was thinking of the New Hollands rated at 235hp if they fit.That way I can afford new, if I go towards 300hp it will have to be reconned.Agree with you about the rudders, and I'll probably convert it to hydraulic steering and get rid of those crappy cables (getting good at steering on gears and revs).According to calcs at 13 tons it should be good for 19-20 knots.The people I'm speaking to reckon theres a problem with cavitation on the 1;1 gearbox setup and with slower bigger props there will be enough horsepower.
Al.

<hr width=100% size=1>No dear,the water goes in the other one.

 

[Micky]

Hi Chaps. As you probably might know i am quite new to motor boating, appart from using boats fitted with an outboard motor such as my Vivacity i have at the moment.

At the moment i am thinking of buying a boat with inboard engine..No, Two engines. It is a 27ft Madeira fitted with two 1.5 BMC engines and they run on direct drive gearboxes on shaft to prop. Cruise speed is around 4-5knts at 1500rpm increasing to 8knts at 2200rpm. This does not seem very much for a "V" shape hull with two engines, hence i thought maybe it would be better to fit a pair of 2-1 boxes and if needed two new props to suit?? The present owner seems to think it would be much better...WHAT DO I DO??? Thanks for all info.

By the way this is a brill site for begginers such a lot of knowledge and very helpful.

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[SP2]

Micky, as people have said - if you change the gear ratios you need to change the stern gear (expensive). Did in several years ago (exchange boxes at Lancing Marine) and got an extra 2 knots. Boat sold the following season!!!

The Madeira hull is not going to fly with the engines - maybe bigger props might help and would be cheaper than the gearbox/shaft etc option. Get a calculation done - I think Vetus can do that but any prop supplier can point you in the right direction.

Finally, older engines (I have a particular 1973 vintage) tend not to suppply quite the grunt they did when nicely run in and still shiny. Shame but true.

Good luck, anyway. Mike

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[Micky]

Thanks Mike.
Would it hurt the engines running the at 2200rpm for long periods of time?

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[SP2]

Not sure what max revs are for those engines. What does the handbook say? Okay, so there is no handbook... Should try Mike Bellamy at Lancing Marine. He will know. Called him once about something and he gave me chapter and verse. Mike

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[Micky]

Thanks Mike, will try him 2morrow.

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[Nauti Fox]

Sorry for drifting off on your thread Micky, sounds like your doing ok on the existing setup.Personally I would leave it alone, unless you have to do major work,is the boat out of the water?ie have you seen the props?
Regards, Al.

<hr width=100% size=1>No dear,the water goes in the other one.

 

[Nauti Fox]

Re: Only 73?

Mike
My venerable lumps of ancient american craftsmanship are but a year older!Although time has finally come for them to be replaced as I just can't get parts for them.
Not on the Medway are you?
Al

<hr width=100% size=1>No dear,the water goes in the other one.

 

[duncan]

wheter it is a planning V hull or not don't know the hull I am afraid) you would seem to have a displacement set up, including the engines, at the moment.
To get beyond 8 knots through the water you are probably going to have to increase power significantly and, unless you can get up and off on the plane, will be burning fuel whilst there!
I have a 26ft boat with plenty of power but I don't run between 7.3 and 9.5 knots through the water as I am just burning fuel to create a wave!.

<hr width=100% size=1>madesco madidum ..../forums/images/icons/smile.gif

 

[pheran]

I have an Owners Manual for the Wortham Blake Fisherboy which is a BMC 1.5 variant. This gives max. rpm as 4000 altho' I personally wouldn't try to get anywhere near that figure on what is now quite an elderly engine.

<hr width=100% size=1><font color=blue>A convert to steel boats and displacement cruising - the only way to go</font color=blue>

 

[Chris_d]

I would forget it, some Maderia/Elysian 27's were fitted with twin outdrive petrols of about 120hp each. I believe these could do 20 odd knots.
However the max hull displacement speed will be about 7knots, to get from 8knts to about 15knots when you will be planing requires a lot of power to climb over the "hump", you are never going to do it with out a substantial increase in horsepower.
The only way to get a displacement boat to go faster is to get a longer waterline length. Basicaly a small planning type v-hull cannot curise efficently at all between about 7 and 15knts, so you either have to get something with more power or be content with 7/8knts max.

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[SP2]

Re: Only 73?

Hi, Al. Yep we are on the Med at Chatham Maritime.

Spectrum Too has two Sabres - one is the original fitted in 1973 although Registration Document puts the engine year as 1971. I like to think they took their time to build the boat...

The other has a factory recon block because the original died in the Channel in 1999 (last owner's problem, not mine). Cost him £5,500 and the wonderful engineers bolted back all of the old ancilliaries (heat exchanger, intercooler etc) so I could spend another 3 grand last year.

I suspect the 1999 problem was that you could not get to the engine easily from the wheelhouse because of furnishings so oils and water were not checked. Personally brought up to check before every trip - the furniture went in the skip when I bought her.

Original sabre smokes dreadfully but runs like a dream (should I have said that?) so I am not worried. Not sure about my raggie neighbours, though. Looks can be so hurtful.

I see you are a Laguna 11.5 - dream boat of mine from the 70's when we cruised my father's mobos out of Medway Bridge. New engines - ouch.

Mike

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[Nauti Fox]

Re: Only 73?

We're down the river a bit at Cuxton,the problem is we like the boat so much that we feel that replacing the engines is the best way to go.Previous owners did'nt use her very much, we do.It seems every time we go out theres another niggly problem.Time to bite the bullet and make her new again!
Regards, al.

<hr width=100% size=1>No dear,the water goes in the other one.