A 27 footer needs engines!

[bc575]

Hello,
I am building a 27 footer in a local Fiberglass boatshop. The owner suggest 150 hp OB and a backup OB of around 20 hp that can not be fitted in the well next to the main engine. It will be installed on one of the bathing platforms. Another option is to go for a twin engine OB. My question(s) is/are:
- Will 150 hp be enoungh? this is a boat that should take 6 persons for a whole day cruise and has a front cabin. With the people, supplies, water, fuel etc.. will a50 hp be enough? taking in consideration it will be mainly used in river water and potentially sea.
- If we go for the twin engine, how much power is needed for each enginr?
- Which option is better? and is 20hp for the backup engine enough?

Thank in advance for your reply....

[gonzo]

What did the designer suggest?

[bc575]

There is no designer on board as such, they use predesigned molds. He gave me both options and did not recommend any. As for the 150 hp capacity, he said they have built the boat for customers in different powers starting from 150hp up to 300 hp.

[Dutch Peter]

Is the boat able to plane or is it a displacement hull? This makes a big difference in the required power!
For a displacement hull you can use the rule of tumb: 7 HP / tonne (for outboards).

[gonzo]

It seems you are asking for something that either the designer can answer, or that it would take a lot of calculations from the plans and lamination schedule. Are you building a boat without specifications?

[bc575]

Hello Gonzo, The specs I have from the builders are :

Dimensions:
L.O.A 850 cm.
Width 295 cm.
Height 140 cm.
Draft 50 cm.

Specifications:
BAKIR CRAFT 850 SPORT is a speed boat with planing
hull design that can accommodate single or dual
outboard engines. It has the following standard
equipment and parts:
- Manual Steering System (or hydraulic stg. is
optional).
- Windscreen.
- Pilot seat.
- Seatings for 8 persons.
- Fore cabin with V-birth.
- Lavatory (including portable toilet and manual water
tap).
- Integrated aft swim platform.
- Stainless steel grab railing on both sides.
- Stainless steel fore grab railing.
- 3 Stainless steel cleats.

Optional Parts And Equipment:
- Outboard Engine
- Batteries for engines and for service
- 3 Navigation Lights
- Lavatory with Portable Toilet and manual water tab
- Electricity shore supply connection and batteries
charger
- Hydraulic steering
- Stainless steel Fuel tank 100 lit.
- Goal Bracket
- Electric Horn

As you see the engine part is left undecided and two options were given 1)dual OB either 70hp+70hp or 90hp+90hp. and 2) one OB of 150 hp capacity and a backup that was not defined.
My problem is to to decide which of the two options is more viable, also if there is some kind of formula like the one suggested by Dutch Peter for th displacement hull (7hp /ton).
Thank you..

[bc575]

Hello,
I have updated my document to reflect the boat specs, kindly have a look at it, and Yes it will be able to plane.... Thanks you.

[gonzo]

In an earlier post you said it was rated up to 300HP. If it can only take 90+90 or 180Hp total it is a big difference. A backup or "kicker" is usually for trolling or emergency use. 15 to 20% of the main engine power is what most boats have.