Max engine size?

micheal · 16 June 2002, 00:00

I have an early 90's BWM deep sea 18 in sound condition with no stress cracks etc. It has a Merc 135hp fitted at the moment which gives 48mph trimmed out on a flat day. I would like to fit a 175hp engine but worry that the boat might start cracking up.

Has anyone had any experience with a similar setup? What would be the max. hp I could go to? Are two engines totalling the same hp necessarily faster?

Would be greatfull for any advise. Thanks.

batfalcon · 17 June 2002, 05:40

Two engines make the boat more stable. In fact they give a train like behavior. They only have 2 disadvantages. 1. They cost too much to buy and 2. They have a BIG fuel consumption, so you must prefigure larger tanks. As far as the 175, I think that it must be bigger than the manufacturer's recommendations and usually overpowering a boat is not an easy (safe) task. I own a Bat (5.3m), which has 115 recommended max hp and I use just that. I' ve read a test some 6 - 7 years ago with the same boat but overpowered with 200 hp. They said that the boat performed exceptionally well doing many many knots, but each manufacturer follows his own rulls. None of us can say how your boat will behave.

John Kennett · 17 June 2002, 08:37

I don't know what that hull is rated to take, but I suspect you are already overpowering it.

There is no way I would put a 175hp on that boat!

John

micheal · 18 June 2002, 20:20

Thanks for the advice chaps.

Batfalcon: Think you are right about the weight & stability. As for fuel consumption, I once read that Mercury reckoned on 1 gal per hour for every 10hp at flat out. I'm not sure with the smaller engines but with larger engines I've owned this seems to be a pretty good rule of thumb. (This only applies to older engines not Optimax's etc.)

Therfore two 60hp's should use the same as one 120h.p roughly speaking?

John: I suspect that the boat would have been rated at 150hp, although it is not plated anywhere showing this. I say this as the build quality is more like a commercial RIB, 1" glassed over ply deck with lots of supports under it, transom with large knees on both sides that after my 3 years of hard use still show no signs of cracking, transom is also almost 3/4" thicker than a Ribtec 645 I veiwed at the weekend rated at 200hp.

I'm not disagreeing with you but would like to be sure about this as I have a limited budget & need a tad more speed at times but don't want to change the boat which I love & has served me well if I don't need to.

Thanks again for the advice.

Pete7 · 18 June 2002, 22:34

Micheal, I think you will find the combined power of twin engines are only equivalent to 75% of a single outboard. So twin 40s are about 60 hp and twin 60s are about 90 hp. BIBOA used to use this rule in the early days to try and match boats into classes for racing.

Your BWM orginally had 2 x 40 mariners fitted which were a disaster as the boat only reached 18 - 20 knots with divers. The REME then fitted a cheap Johnson 150 V6 which subsequently blew up.

Look carefully at the engine weights before going down the twin engine route, you could upset the balance of the boat with big twins. Early BWMs had twin 0.5 inch longitudinal marine ply stringers glassed in with a 0.5 or 0.75 inch deck glassed down on top and not alot else.

You really need to talk to Paul Lemmer who used to run BWM. His e mail is: paullemmer@ribnet.com Think he does brokerage and consultancy now. Pete

John Kennett · 19 June 2002, 07:11

Quote:

You really need to talk to Paul Lemmer who used to run BWM. His e mail is: paullemmer@ribnet.com

In no way related to RIBnet by the way!

John

Alan · 19 June 2002, 13:29

I'd probably be not tempted to go more than 150 on a DS18. I knew a chap who had 115 and he found it more than adequate. Whats wrong with 48 mph anyway?!

Alan

Daniel · 20 June 2002, 13:13

Nice to hear from someone with a similar set up.

I bought my BWM DS 18 2nd hand just after christmas, and am really pleased with it. It has a 130 Yamaha and i have seen 52mph on the GPS 2-up with engine trimed right up and feeling very unstable!!!

With engine trimed down i can make about 40-44mph. Anyway, back to the matter in hand...

I was speaking to Paul (previously BWM) about my / our / this boat at RIBex a few weeks ago, asking lots of questions and generaly trying to extract as much info as i could. One of my queries was the max HP (i, like yourself, am always seeking more speed) and he said 150, although he did smile when i said i had 130 his comment was somehting like "a lot of engine for that small boat". Don't forget, the HP rating is mostly govened by the weight - so check specs before changing anything - for example a 4-stroke 150 will be far too heavy for this boat.

Out of interest how unstable does the boat feel with the engine trimed up - mine is not overly happy in anything resembling a chop???

Let me know the results of any changes you make!

Regards

Daniel