65' Catamaran - How to power it? High thrust outboards

[lwgallant]

Hi guys, I'm relatively new to the boating business and am currently in the process of reviewing different design plans and speaking with various builders about my options. My business partners and I are planning to run a dive charter boat on 7-10 day trips and we need a big boat to do it with! We are currently exploring the idea of a 65' James Wharram Designed "Islander". We will not be under sail power at all, in fact, we will not even have sails at all. We were told that twin high thrust 60hp outboards would be adequate to power the boat. Boat weight is roughly 10 tonne, plus 5 tonnes of guests, food, water, fuel, etc. 15 tonne total. Does anybody know if these engines are enough to power the boat at a decent speed (8-10kts), and what the fuel consumption is like?
Thanks,
Lee

[Pelagic]

How rough can the passages be ...to and from dive sites as outboards are more exposed?

[Cotemar]

You will be better off with two 75 hp diesels. You will only use one of them at a time to get to your dive spot. Two engines only needed to dock an anchor.
This will give you a safe boat. You will have power for navigation systems and dive compressors. Just put the props in tunnels if your worried about draft

[lwgallant]

Hi guys - thanks for the replies.

Pelagic - it will be mostly in the Visaya Region of the Philippines. However, some will be in the Sulu sea which can be rough.

Cotemar - thanks for the recommendation. These diesels would probably be more fuel efficient as well, right?

[Pelagic]

Originally Posted by Cotemar

Just put the props in tunnels if your worried about draft

Cotemar and I are on the same page and the protected props are an added safety factor if picking up drift divers

[a64pilot]

I had a 21' center console with a 175 HP Mercury Verado, boat weight about 6,000 lbs?. Best fuel consumption was just above idle and returned about 6 NM per Gl. On plane at 20 kts, it's 2.3 MPG.
I now have a 30,000 LB sailboat powered by a 44 HP Diesel, at 7 kts and 2,000 RPM my burn rate is 1 gl an hour which of course is 7 MPG.

I'd say the Diesel is more efficient as the whole boat, drive train and engine are optimized at hull speed.


One thing that I think makes a rather large difference is I believe the 44 HP Diesel is turning an 18" prop and the much larger, higher HP outboards turn much smaller diameter props, which I think means they are designed with much higher speeds in mind and I assume these small props just can't deliver high thrust at low speeds which is what we are after.

[Cotemar]

Originally Posted by lwgallant

Hi guys - thanks for the replies.

Pelagic - it will be mostly in the Visaya Region of the Philippines. However, some will be in the Sulu sea which can be rough.

Cotemar - thanks for the recommendation. These diesels would probably be more fuel efficient as well, right?

Diesels will be far more fuel efficient. More reliable and dependable

[Pelagic]

Originally Posted by lwgallant

Hi guys - thanks for the replies.

Pelagic - it will be mostly in the Visaya Region of the Philippines. However, some will be in the Sulu sea which can be rough.

Hi lwgallant... I know those waters very well and have cruised and dove them extensively.
Those seas can get very steep/sharp and confused due to conflicting tides

Don't even think about outboards. Too exposed to weather to properly run and maintain on a 10 day charter.

You will have enough weather delays as it is ....without adding to the problems due to a poor propulsion choice.

[downunder]

Originally Posted by Pelagic

Hi lwgallant... I know those waters very well and have cruised and dove them extensively.
Those seas can get very steep/sharp and confused due to conflicting tides

Don't even think about outboards. Too exposed to weather to properly run and maintain on a 10 day charter.

You will have enough weather delays as it is ....without adding to the problems due to a poor propulsion choice.

=1

[Pelagic]

What design speed and published dive itinerary are you trying to package?

Are you factoring 3 or 4 dives a day?

Most operations that are just beginning tend to underestimate the logistics.

[chrisjs]

Are you able to get diesel outboards where you are?? Would not consider gas OBs but diesels might make sense, though they are heavier. Figure you could lift them for safety when divers are in the water. Could also run the props in tunnels like lobster fishermen in Maine. In terms of power, 2x60HP should be adequate for the proposed vessel. At 15 tons it is still lightweight compared to many similar sized power vessels.