Inboard/Outboard Motor Size

[yogiysh]

Hello Everyone,

I have come across two IFboats without inboard motors but uses an outboard in a motor well in the lazarette or transom motor mount.

I originally had my mind set on an inboard but with further investigation of costs to recondition or replace an inboard diesel as well as tightening environmental pollution control, a 4-stroke outboard motor is starting to look more attractive - but I am still in two minds. :?

Firstly, from your experience how much horse power (HP) is needed to efficiently push an IFboat to hull speed? The IFboats I have seen have inboards with power rating between 4 - 15 HP and outboards 5 - 10 HP. I need to find the right balance between HP, weight and cost. :?

Secondly, what are the pro's and con's between inboard motor vs outboard motor? What I had in mind of is to use the IFboat 40% regattas 60% coastal cruising.

Your advise will be greatly appreciated. :)

[maja]

A boat with an inboard motor is more or less designed for cruising only. Such a boat it is not possible ro race in one design races with other IF-boats.

A modern 4-stroke outboard motor is heavy compared with the older 2-strokes. So the weight will practically limit the power. Also, as the boat is not possibly speedning more than displacement speed, any engine variants with higher power reached by higher RPM are waist of money. So the max. RPM should be around 4500 and it is important to choose a propeller with the lowest pitch. Long rig is a must to help the propeller to be kept in the water.

My opinion is that a 4-5 Hp 4-stroke engine is a good compromise. The fuel consumption will be only 1 litre/hour or slightly more which is good for the environment. At no or very light wind conditions the IF-boat will reach about 5.5 knots, and in moderate heading wind, just below 5 knots. The weight for such a motor will be 23-28 kg, which will still be handable.

I have used a Yamaha F4 on my boat for 7 seasons now and I am quite satisfied. The only things that could be little annoying are some vibrations on certain RPM:s and a rather high noise level. I chose it for it having the lowest weight for this class of engines, 23 kg with long rig. So it is a minor problem to lift it off when racing.

[aadewar]

We cruised for 7 years on our first IF using a pair of 8 foot oars. The boat sailed well enough on even a whisper of wind, and the oars did the job in still conditions. I know some folk with 3 hp motors who are satisfied. I may try a 2 HP on our new boat.

[maja]

The market selection of very small 4-stroke outboards is limited. I think you may only find 2,3 or 2,5 Hp. Some of them cannot be bought with a long rig.

Anyhow, those engines are mainly designed for small inflatable dinghies. Considering the fact that the propeller efficiency will be about 50 % of the rated power, there is not much left for propulsion. The IF-boat needs about 1 Hp of propulsion to run 5 knots in conditions of no heading wind or waves, so I believe such a motor will not bring the boat up to displacement speed very often.