FPB Series

A New Paradigm for Cruising

Design Objectives

FPB 64

FPB 83

FPB 83 On Deck

FPB 83 Interior

FPB 83 Systems

Passages

How Things Are Working Out

Props - Part 2

As mentioned in the previous article, finding the right prop (or props) is a matter of picking the speed and point of maximum efficiency at which we want to operate. Beyond that, at this time in the project, the rest of the parameters are built in (shaft angle, tip clearance, blade immersion, etc. were tied down over a year ago).

Our original concept was to have two sets of props, one with "normal" pitch which we would use most of the time, and a spare set with flatter pitch for use when we expected heavy going.

There were two problems with this approach: First, it is often hard to predict well in advance if we are going to need to punch through big seas. Second, if we damage one prop and want to put on the spare, it would not match the undamaged prop. Running two engines with different propellers causes all sorts of problems, so this means running on one prop, changing both, or hoping the problems of running with different props don't cause damage to the engines or transmissions.

Then there is the matter of propeller design. There are as many propeller designs as there are manufacturers - most have numerous types in their inventory. Blade shape, sweep angle, curvature, and a host of other critical design elements vary. The array of propellers from which we can now chose has been made even more difficult with the advent of computer controlled machining. It is relatively easy to create and build new designs.

We've been wrestling with the propeller decision for a year. A few weeks ago it was time to make a decision, and place our bets. Real world data is hard to come by. Different suppliers have their own computer models to predict efficiency and performance, but we have no way of quantifying their data.

Having talked to most of the top prop designers and suppliers, we eventually came to the following conclusions: 1) As this is a powerboat, and the engines will be running all of the time that we are underway, we want to have the most efficient propeller possible. 2) We also want the quietest props available (we expect the engines to be virtually silent - prop noise is something else - which is why we've got so much tip clearance and such heavy scantlings in the way of the drive shaft supports and plating). 3) While we figure our normal cruising speed will be 11.75 to 12.5 knots, we want to be able to absorb max power at slower speeds when slugging through big seas. 4) Top speed is not as important as being able deal with the heavy weather.

At some point we had to pick a supplier. We ended up working with Mark Power at Henley's Propellers in Auckland for several reasons. First, Mark's suggestions seemed to jive with our own calculations, and he looked at the data in a number of different contexts. Second, he appears to have a very high quality product, one which is probably a notch above other vendors. Third, we liked the characteristics of his "Tiger" designs. Next, the fact that Henley's is local, means we can quickly adjust the pitch should that prove necessary (almost a certainty).The final part of this decision came in talking with several builders and designers who use Mark's Tiger props. They all reported significantly better performance (in terms of speed, noise, and fuel economy). We should add that after all the discussions, discount extensions, and exchange rate calculations, we are still paying about 50% more for these props than if we had purchased them from one of our other potential vendors.

Propellers can be acquired in a variety of materials. The strongest is Nibral bronze, which is what we are using. Then there is the specification on finish and shape tolerance. The standard to which these propellers are being built is the ISO484/2 Class 1. The only higher spec is Class S, which is used on naval vessels.

Props are complex in shape, and making sure they are correct is a critical factor. Henley's (and many other major companies) use something called Prop Scan Technology to check their accuracy. We think this is vital.

During initial sea-trials we'll start out with the boat lighter than she will ever be again - before any personal gear is on board. We want to see just how fast she'll go (which gives us a good data point for future projects). The Deere engines we're using have mechanical fuel pumps and governors, which means they will run 10 to 15% faster than rated (150HP at 2500 RPM). During our lightly loaded trials we'll be able to use this over-speeding for short periods to check our drag figures at higher speeds (and see how fast the hull is capable of moving with these two very small engines).

At the end of trials, Deere will want to see that the engines can turn up to their rated RPM with the boat at normal full load. We should be able to do this with ease.

And then we'll need to find some nasty conditions - normally not too hard in New Zealand. With the boat loaded and blasting through big seas it will quickly become apparent how much the crew can stand, and what is practical from a final propeller pitch perspective. If our bodies can take the punishment (the boat will have no problems in this regard) then the flatter pitch, which allows pushing the boat in these conditions, will stay. On the other hand, if it turns out we have to slow down because of our physiological requirements, we'll add some pitch to the props, so the engines run a little slower at normal cruising speed.

One last detail. The photo below shows a detail of the back of the prop hubs we'll be using. There are tapped holes in the hub. When the time comes to remove the props, all we need is a simple puller (supplied by Henley's) and a couple of bolts. Much easier to use - especially underwater - than the unwieldy prop pullers to which we are accustomed!