Over the past year we have posted photos of various craft and their bow and stern waves as an indicator of efficiency. This is judged in terms of Speed Length Ratio (SLR) or the boat speed divided by the square root of the waterline length. In the photo above, sent to us by a cruiser in Scotland, Wind Horse is doing her normal eleven knots. She has an 81 foot waterline, the square root of which is nine. Dividing eleven knots by nine gives us an SLR of 1.22.
As you can see from the photo above there is not much bow wave (and we are full load here or about 44 tonnes/92,000 pounds).
The same is true aft. A nice clean release off the transom with little magnitude to the stern wave.
Virtually all commercial vessels and displacement yachts operate at a SLR of around .9, or for Wind Horse a little over eight knots. This would be more efficient for Wind Horse in terms of mileage, but because of a variety of factors we have the option of cruising at eleven knots, in a vibration free and quiet environment. We can do this for in excess of six thousand nautical miles because of the factors which we have described in other articles on the website.
To recap these critical factors for efficient cruising include:
- Waterline length.
- Displacement.
- Correct center of buoyancy.
- Distribution of volume (curve of area) along the waterline.
- Moderate maximum sectional area and its location.
- Waterline beam.
- Wetted surface.
- Appendage drag.
- Prop efficiency (including water flow to and around the prop).
- Drag caused by sea state.
- Windage.
Wow, a long and intimidating list. Making things even more complex is the fact that all of these issues are interrelated. Adjust one and it affects others. When you start to factor in comfort at sea, and heavy weather capabilities, a whole new set of parameters comes into play.
Back to SLRs. Realistically, most heavy displacement trawler types cruise long distances at SLRs of the aforementioned .9 to a maximum of one. For a yacht with a 64-foot waterline this is about 7.2 to eight knots, or 173 to 192 nautical miles per day. Yes, you can pour the coals to the prop and go faster, but then you are not going to go very far, and it will be noisy.
The FPB 64 is optimized to run at between 9.7 knots (233 nautical miles per day) which is an SLR of 1.21, about the same as the FPB 83 Wind Horse in terms of speed length ratio. And she has the range to go round trip across most oceans.
Posted by Steve Dashew (September 7, 2009)
September 8th, 2009 at 11:29 pm
Dear Mr/Mrs.D.
You mention (FPB 64/engine room) that windhorse hydrostatic characteristics dictated 2 engines but the 64 will be ok with one. Can you elaborate in plain english why the 83 needs 2 engines.
keep us posted on the 64. Enjoying every part of the construction. Its an obligated web daily check. When is the next DVD coming out? Thank you
Carlos
September 9th, 2009 at 2:41 am
Hi Carlos:
There is a tradeoff between single and twin engines. Twins have a maneuverability advantage, offer redundancy, take more space in the engine room, are less efficient than a single screw, and are more vulnerable to ice and debris.
The single screw approach is more efficient, and better protected.
Our preference has always been single screw. This requires a larger diameter prop to get the power into the water than is the case when the power is split with two engines.
To fit the prop you start with clearance between the hull and prop blade tip. Then add shaft angle (the amount depends on the hull shape). Hull shape also controls the angle of water flow to and around the prop. Excess angle between the prop and hull creates a series of drag and propulsion problems.
The FPB 83 is lighter for its length than the FPB64. As such, the hull (really the canoe body without fins) is shallower than the FPB 64. The shallower hull would require an excessive shaft angle in order to fit a single larger prop. Hence we are forced to use twins on the FPB 83. The FPB 64, on the other hand, has a hull which is deep enough to fit a single prop and still maintain good tip clearance and an efficient prop shaft to hull angle.
September 10th, 2009 at 12:37 am
Hi Steve
Thanks for your detailed explanation. I keep learning good “makes a lot of sense” stuff from your writings since I start reading about the unsailboat. .
Ill try to visualize why a ¨shallower hull would require an excessive shaft angle in order to fit a single larger prop¨ .
Is prop aeration common in Windhorse?
Thanks again
Carlos
September 10th, 2009 at 2:58 am
Carlos:
Aeration and/o cavitation is NOT a problem with Wind Horse. But concern with this potential is one of the reasons we did so much work with CFD analysis in the early stages of the design. Even under the most adverse conditions we never feel vibration from the props cavitating. We also have the option of shifting fuel and water aft to sink the stern, but have never needed to do this.