Re-powering Bertram 42

Just curious - why no interest in rebuilding the existing engines? How far gone are they?

Maybe this is of any use:
1) With a total length of 42 feet, the waterline will be more like 38 feet.
2) Not a big difference but the hull speed will be 6,16 x 1,34 = 8,25 knots.
3) Best efficiency / speed ratio 6,16 x 1,2 = 7,4 knots
4) If engines are swapped for lighter ones ( in weight ) you could consider to add ballast
which you could place actually lower in the bilge.

Calculation of power needed. This is total power at the 2 propellers and engines.
For 8,25 knots – prop.hp: 88 hp – engine hp: 94 hp total
For 9 knots – prop.hp: 103 hp – engine hp: 110 hp total
For 12,55 knots – prop.hp: 283 – engine hp: 300 hp total

Amazing that for the hull speed of 8,25 knots only 94 hp or 47 hp per engine is necessary. A safety margin for wind and waves add 100%, so the total power
is then (rounded off) 200 hp total or 100 hp per engine.

If you go this route you can choose for a (medium-) slow turning 1800 rpm
5 to 7 liter engine without turbo and aftercooler. The weight is about half of the weight of your GM engines and they last “forever”.

So far the calculations. Note that from 8 knots to 12 knots you need 3 times the horsepower, meaning you burn 3 times the fuel.

Personal experience: with my 22.000 kilo (loaded) catamaran with the sails down I always run 1 engine at the time. With a use of 3,25 liter per hour the speed is 5 knots.
For 6 knots it burns 4,6 liter per hour. (When I want to go faster I wait for good wind and I go 12 knots and do not need an engine at all, but that is not relevant here).
With the propellers 6 meters (!) apart I have to use 2 engines moving in marina’s and anchorages under 1,5 knots. Over that speed the rudders take over the steering and cruising they are slightly steering to one side to compensate the effect of the (one sided-) engine.

That practically all powerboats are overpowered makes interesting coffee-table talk.
( like talking anchors….)
An educating web-site is: Vicprop - Prop calculator for Displacement and semi-displacement hulls

Andrew- a few options here: Since you do not have far to go to fish, fuel economy traveling is not a big deal. Rebuilding the 692's may be one option, and probably the lowest cost. Depends on what is wrong with them.

Another is to repower with small engines like the Cummins 6BT 220, or a similar Euro engine popular in your market. Cruising at hull speed around 8kts will not use much of the 220hp, but will load them enough to be happy.

Another is to repower with something like the Cummins 6CTA 450, which will allow the boat to scoot and preserves resale. This engine is more expensive than the 220, but you will gain that on resale.

The 450 will only burn a little more than the 220 at same speed. Guessing 0.5gph more, each.

That hull is not very efficient at low speed compared to one designed for slow.

RickB said:

A string of folks telling him what he wants is not what they want or would do is no help to anyone.

Click to expand...

Oh but Rick, the more information the better. You never know he might change his mind.

Powering to displacement speed makes sense, any thing more Unless you want to plane is senseless.

On my semi-displacement hull I avoid speeds just above displacement speeds, the speed just where the bow starts to rise. As Ted mentioned above you are just pushing water.

Perhaps the OP should go to a re-power specialist in his area and get some advise. I would be interested to hear what they say.

timjet said:

Oh but Rick, the more information the better. You never know he might change his mind. Powering to displacement speed makes sense, any thing more Unless you want to plane is senseless. On my semi-displacement hull I avoid speeds just above displacement speeds, the speed just where the bow starts to rise. As Ted mentioned above you are just pushing water. Perhaps the OP should go to a re-power specialist in his area and get some advise. I would be interested to hear what they say.

Click to expand...

A fishing boat needs to do ten knots in reverse. It also has to have sufficient power to safely maneuver with 12000 lbs of water sloshing around in the cockpit. Take a look at this and then decide if 200 hp is enough;
Via iPad using Trawler

cafesport said:

A fishing boat needs to do ten knots in reverse. It also has to have sufficient power to safely maneuver with 12000 lbs of water sloshing around in the cockpit. Take a look at this and then decide if 200 hp is enough;

Via iPad using Trawler

Click to expand...

A lot of pretty savy guys think that backing on a fish is just stupid and dangerous...there are other tactics proven effective to gain on a fish.

More than one owner/capt has sunk a boat , hurt crew or trashed an engine by backing down on a fish...not sure that I would ever do it to my boat.

Resale isn't for everyone...some of us have factored in original cost, cost of improvements and maintenance...and see that in the long run if the boat is a giveaway/throwaway...so be it.

So someone that got a Bertram 42 for a song and want's to use rubber bands to power it for a few years then unload the hull to someone else or the dumpster...I can see the logic...actually someone might pay good money for the hull if the boat is in decent shape after another decade no matter what is done with the current engines.

Imho....
I would never forgive anyone who would do that to a Bertram. I would sell it and buy what you want/need. It will never work well, as it was designed for a whole different purpose....and well designed they are.

A lot of great information to chew on for the OP. I think the backing down vid is nice for tournaments where tagging and releasing quickly could be the difference between winning and loosing, but in reality lightly backing down and maneuverability is most important to keep your game out of the running gear.

So far the 450 Cummin's is the best advice I've heard so far.

Why not rebuild

twistedtree said:

Just curious - why no interest in rebuilding the existing engines? How far gone are they?

Click to expand...

I would be willing to bet it would be far less expensive to rebuild these Detroits.

Rebuild kits

Detroit Diesel 6V92 Turbo | Rebuild Kit | Overhaul | Inframe Rebuild kits for Detroit 6Vturbos less than $2000 each.

I see that you are from the UK.
Go on www.boatdiesel.com and ask for Paul Foulston's help. He is a Cummins guru in the UK. He takes a very common sense approach and has done some "different" things than the norm from his stories.
I'm sure he can offer advice re the Cummins 220 in your application.
Plus he can offer some local help when the dirt starts to fly.

Haven't read the whole thread but has there been any consideration given to modifying the shape of the stern? A FD hull could be made with considerable cutting, laminating and reconfiguring the stern. Single or twin w new prop shaft sett ups and rudder or rudders. Stability and trim would need to be considered of course as this is just an personal idea of mine.

I think it would perform about 80% as good as a regular FD boat depending on the conversion and the FD boat compared to.

If given big rudders I think it may be a practical conversion. The guy to pull it off easily would be Hendo 78.

A DD series 50 sadly is electronic , but would have the power to maintain the resale and is not fuel inefficient as the 2 strokes are at light loads.
Same tranny , drive shaft and prop.

Cheapest for you might be to get the current engines rebuilt and install far more economical injectors.

The next owner could install de fuelers if desired.

The 2 strokes are economical at above 60% of the power there built to , which might still be higher than you want , even tuned down.

Check with a local small charter bus co, and see if they can recommend a rebuilder.