To A New Paradigm With FPB

FPB 64 – Hull Shape

As we alluded to in the introduction page to the FPB 64, coming up with a shorter hull shape that has comparable comfort and sea-keeping abilities to the FPB 83 has taken us a while.


The smaller boat has a similar range at cruising speed, and carries the same personal payload. That this takes place on a shorter waterline has advantages and disadvantages, depending on how volume is distributed throughout the canoe body (shown above)

Requirements

Take upwind performance.


If some of the extra volume ends up in the forward sections, in the form of hull depth, it can soften the ride.


On the other hand, too much hull depth forward, what we call “bite”, makes the boat hard to steer downwind in even small waves, which limits how long you can run with the seas (and how much fun you can have surfing).


What we refer to as the business end of the boat.


Sea trials have confirmed that bow bite off the wind is not going to be a problem. The FPB 64 tracks nicely, with the autopilot barely working.

In the image below we’ve turned on a rough waterline (green plane), so you can see the hull shape which is penetrating head seas.


The entry angles are slightly wider on the FPB 64 (above) than the FPB 83, but they are also a little deeper, partially offsetting the wider entry angle of the smaller boat.

The waves see the bow in a combination of dimensions leading to the total volume at any location. This includes the plan view (looking up) as seen above, and the profile view as shown below.


What the waves are working on is how the volume is developed as the waves run along the hull. It is the 3 dimensional distribution of volume that determines how the boat behaves in the head seas.


How the volume is developed down the center of the hull, and in particular aft, is also a part of this pitching equation. A wide, buoyant transom is a perfect target for the wave energy, lifting the stern, which then has the effect of shoving the bow down. This is why traditional trawler hulls have such high bows. They pitch so badly that they are forced to add buoyancy forward to try to keep the bow from submarining into the oncoming wave crest.

In the traditional full bowed trawler you now you have all this extra forward buoyancy for the waves to impact. You might as well be trying to drive a breakwater through the waves. This type of configuration hobby horses, slams, stops, and then tries to get going again. Anyone who has been uphill in a trawler type of hull will be able to describe the process for you in detail.


The FPB 64 has what we refer to as a soft shape.


It does not give the seas much to grab onto. So the tendency for the bow to be driven down as the wave passes under the stern is mitigated.

The FPB Approach Upwind

The FPB avoids these problems by having very fine lines forward and aft, with little for waves to grab onto at either end of the boat. The hull knifes through the waves, with little pitching, and most of the time virtually no change in speed. There is just enough buoyancy so the bow lifts on the really big seas, but not so much that the buoyancy is an impediment.


The FPB 64 is, as we’ve described, a little fuller – but still sharp and fairly close to the FPB 83. You would think that she will be a bit less comfortable uphill than her bigger sister, except for one factor – longitudinal inertia (think of this as fore and aft stability). Because of its longer length the FPB 83 has almost twice the longitudinal inertia of the FPB 64 (inertia goes up with the square of the difference in length, amongst other things, so the added length rapidly increases inertia). What this means is that although the smaller boat has slightly fuller ends, and will move around a bit more in head seas, motion is less abrupt, and you feel it less.

We know from our early sea trials that in most sea sates the FPB 64 will be exceptionally comfortable upwind.


The photo above was taken north of Cedros Island off the coast of Baja California (Mexico). The wind is out of the northwest, and it has been blowing in the mid-30s for the past couple of days. These seas are in the 8 to 14-foot (2.4 to 4.2m) range and very steep due to a north flowing current opposing the waves. Check out the crest looming in the background.

The FPB 83 maintains her 11 knots in these conditions, slowing down to 9.5 knots in the really big seas, and then quickly accelerating back to speed. There is a bit of spray on deck, mainly from the impact made by the anchor. But rarely solid water.

Which brings us back to the hull on the FPB 64.


The net affect of these two characteristics – longitudinal inertia and volume distribution – will be a comparable ride to the FPB 83 much of the time, with the FPB 64 potentially having some advantages in certain sea states. The photo above is one of a series available of the FPB 65 on a blustery day, in full screen size by clicking here.

Running Downwind

Early on in this page we mentioned “bite” forward and its relation to the tendency of a boat to bow steer. Deeply immersed bows have a problem going downwind. If a wave pushes the bow or the stern, turning the hull, a deeply immersed bow reacts with forces that tend to accelerate this process. The result is they generate a turning momentum in addition to what the wave energy starts. Some hull shapes actually accelerate this turning process which can lead to a broach. This is why traditional displacement power boats and planing hulls at displacement speeds have such an uncomfortable ride downwind, and why in what we would consider moderate sea-states, they slow down, or turn into the waves and jog slowly uphill, even if this is 180-degrees off course.

Have a look at the series of wave photos which follow. These were taken this fall off Cape Mendocino (Northern California) on a day we looked forward to for some surfing. Keep in mind the camera lens tends to flatten waves and make them look smaller.


This first photo is taken from the flying bridge of Wind Horse looking ahead (downwind).


This is the view looking behind the boat. These waves are not particularly big by our standards. Buoy reports indicated 13 feet (4m) height at 10-second intervals, with occasional bigger sets coming through. However, they are steep, and it is this shape which causes problems for boats which bow steer.


This wave is rearing up, ready to give us a bit of a ride. This is the most fun we have on the boat at sea. It is also fast, engine load drops, fuel burn is reduced, and we get to watch the steam gauge climb. “Kowabunga, Dude!”


Here is a shot of the boat speed on one of these waves. For more information on this particular surfing contest click here.

While the FPB 64 will never surf at the speeds of the FPB 83, we have already seen that she will give a god account of herself in moderate size waves.


She is well behaved in terms of steering control, and will generate surfing pleasure going downhill, just at a reduced endorphin level (this will just take a bit more wind and slightly bigger waves to really get her rolling).

Fins

As you may have now figured out, our bows are shallow and have little tendency to bite. However, the FPB 64 is deeper than the FPB 83, and one of the ways we compensate for this is with a bigger rudder relative to her size.


In fact, this rudder is almost the same surface area as the combined twin rudders of the FPB 83. Because it is centered under the hull, and deeper than the FPB 83′s rudders, in heavy weather this single rudder will be as effective (if not more) than those twin outboard foils of the FPB 83 (and this on a much smaller boat with less longitudinal stability). In relation to the size of the FPB 64, this is a substantially larger rudder, and a more powerful steering device.

There is another advantage to this one large rudder, and its adjacent prop. It will have better steering authority in close quarters maneuvering than what the FPB 83 exhibits (and the bigger boat steers very well). Between the oversized rudder, 42 degrees rudder deflection angle off center, and the short length, the FPB 64 is easier to park in tight places than you might expect.

The skeg plays a part in this as well. Not only does the skeg protect the prop and rudder from most debris, but it helps to balance the bow. In scale, this single skeg will develop more tracking force than the smaller twin skegs on the FPB 83.

The big rudder and skeg also work to reduce prop “walk” in reverse. 15 degree bow offset is sufficient to have you going straight within two boat lengths when backing down.

There’s one other part of this equation, the keel…


…or lack thereof. The keel on the FPB 83 is there for several reasons, one of which is holding the boat in a cross wind when docking. The smaller boat, with its deeper canoe body, does not need the keel for this purpose. Neither does it require it for ballast. Getting rid of the keel is a benefit in terms of maneuvering in port and at sea.

Eliminating the keel moves the pivot point of the hull (about which the boat rotates when turning) forward. This allows the the hull to slide more easily with the turn than would be the case if a keel were present. Allowing the hull to slide (as opposed to pivoting on the keel) helps the bow avoid a positive angle of attack when knocked around by waves (it is this angle of attack on the bow which forces the bow to generate lift and exacerbates the tendency to over-steer into a broach).

Draft

There is another big advantage, draft. At nominal half load, 75,000 pounds (34,000 kg), the FPB 64 draws just 38″ (975mm) at the hull. The bottom edge of the stabilizers, when they are centered, is on roughly the same plane. At the propeller skeg draft is 4.5 feet (1.375m) (and we’re working on reducing this).

Consider cruising in thin water, or making a navigational error. Except for a very small area at the aft end of the boat, in water a little deeper than three feet (90cm) you are home free.


Want to dry the boat out on a lovely beach, or let her sit between tides on a shallow river? The stabilizers should keep you upright (but avoid setting them on rocks). We don’t know about you, but the concept of being able to put the bow onto the beach is very appealing to us.


There is one more consideration, “gridding” the boat. In much of the world tidal grids like the one above (in Cordova, in Alaska’s Prince William Sound) offers a simple means of drying out for inspection and maintenance. The FPB 64, with its clean underbody and robust rub rail, is an ideal candidate for the grid.

For more information on our approach to hydrostatics see the section on the side bar called “Design Objectives”.

If you would like to learn more about the FPB 64 program contact ToddR@SetSail.com.


Posted by Steve Dashew  (October 16, 2011)




2 Responses to “FPB 64 – Hull Shape”

  1. Conrad Says:
    Hi Steve… I like your thinking re the extended skeg but not attaching it to the rudder – I hadn’t considered the rudder damage that could be caused by a deformed skeg. Have you (I’m sure you have) given much thought to other rudder types, such as an articulated rudder?

    [Reply]

    Steve Dashew Reply:

    Lots of thought. Articulating rudders are great, but higher in drag under way and more complex. If you don’t have sufficient span for requisite rudder authority, they are the only option. In our case, the boats are so easily steered that we can fit huge rudders relative to the forces required, and use simple, but very efficient spades.

    [Reply]



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