Predicting performance under power is relatively simple if you have a hull form that fits standard models (the definition of standard here covers a gamut of fishing trawlers to high speed destroyer hulls). In the olden days you would look up David Taylor model test data for something similar and then go to work with your slide rule. More recently, this process has become quicker, with the ubitquitous computer filling in for the retired slide rule. But if you pick the wrong model data, or miss something in your hull characteristics, the resulting calcs can be off, sometimes by a lot.
Tank testing can be a help, but the tank drag data has to be scaled from the towed model to full size, and this process is frought with potential error. Having data on similar models that turned out to tie to the real world is critical to even getting into the ballpark with the projections. A major impact on accuracy is the size of the test model. The bigger the model in scale, the better the results are likely to be. But big models are very costly, so only the well heeled programs, looking for that last bit of performance, can afford anything larger than one fifth size and most are more in the range of 1/10th scale.
Over the years we have accumulated data from our prediction software in the context of a relatively consistent family of hulls, with plenty of opportunity to observe the results in the real world. When we did the design work on the FPB prototype, WInd Horse, we did go into the tank, but this was more to check the release of the stern wave and to see if squatting might be a problem, rather than looking for accurate drag numbers.
Correlating tank and VPP data with smooth water operation is one thing. Doing it for rough water is something altogether different. It is the latter envrionment where real world data becomes so critical.
We’ve got more than 50,000 miles of performance information on WInd Horse so we have a pretty good idea of the power required for normal operation. However, since we’ve never come close to running out of top end, even in the gnarliest of up wind sea states, we have had, in effect, excess power, and so we were without a data set of which we were highly desirous.
As previously reported, we had the diesels on Wind Horse derated by resetting the injection pumps from M4 to M1/3. Prop pitch was altered as well to reduce RPM, giving us an M1 rating of 105 HP at 2300 RPM.
Wind Horse is close to a scale model of the FPB 115, and as such, in effect, a real world test bed, far more valuable than anything we could do in the tank, or with computers. Our goal with the mods has been to get closer to the answer of what sort of power we need for the FPB 115, beyond normal cruise, for difficult sea states. We finally got a chance to test this set up crossing Albemarle Sound today.
Albemarle has a reputation for kicking up a vicous chop when the wind blows. With a post frontal breeze at a steady 25 knots plus, gusting higher, three to six foot (.9 to 1.8m) short coupled waves were making their presence known. In the interest of science, this was enough incentive to get us to move from a snug anchorage just north of the Sound.
Now these seas and the wind force in and of themselves would not be a sufficient test, except fore one other factor – water depth, or more precisely a lack thereof. Depths averaged 15 to 20 feet (4.5 to 6m) during our journey which is shallow enough for the hull to feel an increase in drag. While we don’t have precise numbers in smooth water shallow depths like this will increase drag by ten to twenty percent.
We are pleased to report that Wind Horse was able to achieve the M1 rated 2300 RPM in these conditions, with a speed over the ground of 11.7 knots. Between alternators and stabilizer hydraulics there was a load of roughly ten horsepower per engine, leaving approximately 95HP from each engine, or 190 HP total, to overcome propulsion drag, sea state, and windage. We have more testing to do once Wind Horse has her swim step extension fitted (similar to the FPB64s), but for now we know we are on the right track.
Posted by Steve Dashew (October 20, 2011)
October 21st, 2011 at 5:18 pm
Hi Steve,
I hope you had a great visit to your terra firma home.
I’ve always been curious about scale models in a tank test (or a wind tunnel for that matter). As you’ve mentioned the models are typically something like 1/10 scale, but is the medium in the tank 1/10 as dense as sea water? Also, is the depth of the medium adjusted to avoid bottom drag relative to the model scale?
Cheers,
David
October 21st, 2011 at 9:46 pm
Hi David:
I am far from an expert on tank testing. My understanding is the density remains the same as t he medium in which the hull would float. When the numbers are crunched adjustment for temperature ust be made. The big scaling issues come from the difference in Reynolds numbers, and the intepolations related thereto. Perhaps a SetSail visitor familiar with the process will comment further.
October 22nd, 2011 at 10:03 pm
Are the FPB64 engines set at the M4 rating? If so have any of the owner de-rated them to M1 and increased the prop pitch? Other than power for control in a bar crossing situation is there a real need for 235 HP?
October 23rd, 2011 at 4:55 am
Good question, Daryl:
The answer is it depends. We have plenty of power at t he M3 rating. The past few days Avatar, FPB 64-1, has been caught by a depression on her way from Fiji to New Zealand. The breeze was 25 to 40 knots, gusting into the high forties. Seas are from south and southwest, so confused at 10 to 50 degrees to her course. Boat speed varies between seven and nine and a half knots. And engine loading? Forty six to fifty percent, jumping to sixty when the boat is slowed down by running into a particularly big sea.
The issue of the engine rating for the FPB 64s boils down to this: there are scenarios where the power might be required. Perhaps in the current situation even worse weather is on its way and regardless of motion, you want to accelerate progress as much as possible. Then those extra horses are going to be welcome. Given the Tier ll computer controlled injection pump the penalty at normal cruise is minimal, maybe less than a percent. Where you might want to consider derating is if you were going to slow way down to extend range.
October 24th, 2011 at 2:06 pm
Thanks for the informative website.
Taking into account all the above, have you found a prop calculator that is accurate for your boats?
October 24th, 2011 at 6:53 pm
Howdy Richard:
We have several drag and/or velocity prediction packages we use. None are exact, but we have enough real world data now t o understand how to compensate for these failings, so we can get pretty close.
October 26th, 2011 at 3:07 pm
The whole derating issue confuses me. Does it damage the engines to run at 40% (or less) for extended times? Is the extra horsepower at top end just wasted (that is, if I run at full throttle at full rating does it affect speed or just fuel burn rate)? Can the derating/uprating be done at sea? Why mess with the derating at all?
BTW, thanks for posting all this. If nothing else it feeds my fantasies of one day being able to go out.
October 26th, 2011 at 4:43 pm
John:
Operating load and life span is a complex question and depends on the type of engine. Derating, likewise, depends on the engine. With our Deere 4054 TFMs the injection pump rack settings have to be reset at an injection pump facility. With the tier ll Deere 6068s (used on the FPB 64s) and their electronic fuel injection pumps, derating is as simple as connecting a computer and changing the engine CPU settings. In both cases the same injectors are used. Other engines require pump and injectors be changed.
In our case, we derated as a test for the FPB drive line decision process. The efficiency gains alone, at the most three to five percent, would not justify the cost of changing the injection pumps.