More drive system queries

[Willallison]

All of the various drive systems have their advantages:
Jets offer shallow draft and safety
Shafts give simplicity and ease of maintenence
Sterndrives tend to be more efficient.

But which is the best ? (loaded question if ever there was one
Given a particular vessel - lets say a 4 to 5000kg cruiser of around 35 ft, powered by a single diesel engine in the 400 - 450 hp range. The boat is intended for coastal cruising and would be capable of speeds to around 30 knots.
Jets seem a nice choice. But then you have to consider that the boat may have to be run quite regularly at say 18 to 20 knots and jets tend to be quite inefficient at these speeds (?)
A sterndrive would be nice too - but you'd be restricted by the hp requirement to using a unit such as the Konrad ( http://www.konradmarine.com ). Parts may be a problem if the boat is to be used outside the US and sterndrives aren't too good if kept in the water for long periods.
That leaves the old fashioned choice - shafts. Nice and simple, but a quick browse through a few magazine comparisons and you soon see that the perfomance of a shaft drive boat falls well short of the same boat equipped with a sterndrive.
So which to choose.......

[Willallison]

And then there's the surface drive and its multitude of derivatives, but I guess they don't come into their own until higher speeds

[Portager]

From a purely propulsive standpoint the stern drive should have a slight propulsive advantage in that it can align the thrust vector with the velocity vector and use the thrust for efficiently. This benefit is 0.4% for 5 degrees, 1.4% for 10%, 3.4% for 15 degrees and 6% for 20 degrees of thrust vector misalignment. However, the propulsive efficiency is only a portion of the comparison. By tilting the stern drive some of the thrust can be used to produce a vertical thrust component to lift or lower the stern and a moment to lift or lower the bow. The inboard can achieve the same benefits by using trim tabs (large trim tabs are the tell tail sign of a poorly ballanced design), but trim tabs produce higher drag causing a larger performance penalty. Therefore inboards performance tends to be more sensitive to trim conditions.

I think the performance difference you are seeing between stern drives and inboards on the same model is a function of the designers ability or effort to tailor the mass properties of the inboard version of the boat.

An additional benefit of stern drives is there is no physical limitation on prop diameter and they have dual prop options which provide higher propulsive efficiency. The main weakness of stern drives is corrosion resistance, especially in salt water. The relatively new Volvo Penta Ocean Series XDP composite drive may resolve this deficiency but the jury is still out. The other disadvantage of stern drives is cost. Inboard drives are generally cheaper.

Cheers;
Mike Schooley

[Portager]

You didn't mention the requirement to operate at displacement speeds. If you intend to operate in canals or inland passages with wake limits or in heavy sea states you might need to operate at displacement speeds for prolonged periods. Since this will require only about 5% of your maximum power you will be operating far from engine and propeller optimum or design point.

I think the answer to your question is a controllable pitch propeller (CPP).

With a fixed pitch propeller the engine must operate at the speed dictated by the propeller power demand curve. Since the propeller power demand curve is steeper than the engines optimum specific fuel consumption curve (http://www.sabb.no/images/hvp/e-hvp-t.jpg) the engine will be under loaded at all power levels below the design point and overloaded at points above. This reduces fuel efficiency and in extreme cases is hard on engines. The CPP allows the operator to adjust the pitch of the blades to achieve a better engine loading, which in turn improves fuel efficiency and engine life. See http://www.kastenmarine.com/CPprops.htm and http://www.sabb.no/engelsk/hvp_gear.htm

Controllable pitch propellers are intrinsically more expensive that fixed pitch propellers. However, since the CPP eliminates the need for a forward and reverse gearbox, the CPP drive train can (sometimes) be competitive with a fixed pitch drive train. In general CPP drives cost a little more than fixed pitch propellers but on commercial boats or boats that operate a lot, they can pay for themselves in fuel savings. CPP are also a big benefit on MotorSAILERS (with the emphasis on sailing) since the prop can be feathered to improve performance in the sail only mode.

Cheers;
Mike Schooley