Conclusions re effect of ropecutters on speed

[jfm]

OK here is my amateurish test of ropecutters and speed. A few weeks ago our new boat had its predelivery seatrials. They did several runs and got 32.8 knots top speed on the log. That was a pleasing top speed as the brochure claims 31kts (Sq58 with 715 EVC Volvos). The boat had no ropecutters at this point.

I spoke to the skipper who did the trials. He did them between chichester and south side of IoW, 32 miles in all, in a force 7, and because of the waves he might not have done a very long run to wind the boat up to the last knot of top speed, but he did 6 runs in all and got 32-33 knots each time, and averaged 32.8. He got 2350 max rpm. He wrote all this up in his official report on the sea trials of which I have a copy. The trials were done when the engines had about 5 or 6 hours on them.

Then after the trials the boat was lifted out and we had rope cutters fitted, "Strippers" with 2 blades each

Then we took delivery a couple of days ago and I got to see what speed it would do. Our speeds with the ropecutters were done on the same load of fuel as the sea trials, the same rough loading (we had 2 or 3 people on and full water and 3/4 fuel, the sea trial had 2 people and full water and full-7/8 fuel). Also the bottom was 100% brand new clean throughout, and the low temperatures gave top performance from the engines.

After a further say 5 or 6 engine hours (so maybe the engines had freed up a touch) we did trials and got 2350rpm and an easy 32.5knots on the log. When we allowed it to wind up for a few minutes we got 33+ just.

Hence I conclude that these rope cutters make no significant difference to speed, and certainly not the 2 or 3 knots you hear about in some write-ups.

By the way, I should add, we did most of the 10 or so hours we put on th engines in the last few days at 16-1700rpm as they are new engines, the runs at max rpm 2350 were pretty short and at full eng operating temp. I'm not an engine thrasher!

(PS, Been in hurry today, will do a proper write up tomorrow of first channel crossing in new boat, midchannel failure of EVC lectronix, person landed on our flybridge by coastguard helicopter at 20kts, and loading onto transport ship. Will post a few pics too)

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[longjohnsilver]

Boring!

midchannel failure of EVC lectronix, person landed on our flybridge by coastguard helicopter at 20kts, and loading onto transport ship.

So all pretty routine and uneventful then jfm? ;-))

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[tcm]

oh dear, unfortunately the max revs is 80% for first 50 hours so the engines are now shagged and not run properly in as stipulated in the manual which of course you are duty bound to tell the next owner. Ah well.

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[Deleted User YDKXO]

Mmm, not sure this is scientific since effect of F7 waves depending on direction is sure to knock speed off but most impressed with fact that boat can be maxxed (no pun intended) out in these conditions. Must be pretty good hull. Which way was wind coming from?

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[mjf]

I am curious, why are you using the 'log' and not GPS data?

Michael

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[tcm]

I bet it's a real log as well with the knots evry 84 feet or whatever, he is such a Luddite.

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[jfm]

log vs GPS

Becos it was tidal where I was, springs too, and the log seemed to work ok, was new and clean etc, and I had both log and GPS data from the predelivery seatrials but of course the GPS was affected by tide though I dont know how much.

The log had been calibrated as part of the PDI and was therefore correct I believe. The GPS and log said more or less the same thing when running across tide and in harbours.

And I'm a luddite. I even used a paper chart last weekend, though only once

Frankly I find the conclusion that the ropecutters make nearly no difference entirely plausible. When you look attem, compared with the p brackets and the hub of the prop, etc, they're tiny. You hear it said that cutters cost you 2-3knots top speed but they dont look anything like 2-3 knots (which is 200bhp at least) worth of drag.

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[kimhollamby]

Good, but not the whole story

We got info on a whole bunch of tank test trials on ropecutters way back which found that:

a) they decreased speed in some cases
b) they had no discernible effect in some cases
c) they actually marginally increased speed in some cases

In a couple of cases they even generated cavitation burns in bronze props near the hub.

My feeling is there really is no hard and fast answer for planing boats -- it's a bit pot luck depending upon how the waterflow is presented to the props and the shape of the props too I guess.

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[mjf]

Re: log vs GPS

Interesting, but you are comparing speed through the water to speed over the ground when using log vs GPS. The cross tide data is prehaps the best indication.

I think maybe on 58ft the slight adjustment to the shaft length (position of the props in realtion to the hull) is small compared to smaller craft. There is the HP issue too.

I was thinking of retro -fitting these beasts myself but was worried about the side effects. Dry Suit and small air bottle / regulator is the other alternative.

When I hauled my boat two weeks ago there was the remnants of a bucket (handle only left!) swinging around the shaft by the P Bracket. This would have severely damaged the rope cutters, no?

Michael

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[jfm]

Re: log vs GPS

Not sure i understand your point in your first sentence. Whether log is correct or not it was the same log before and after the ropecutters were fitted, so the speed results taken from log ought to be comparable, no? SOG has nuthin to do with any of this, unless you are 100% sure there is no tide on both the before/after runs.

Yes, ropecutters get busted by debris and need re-fettling or replacing. On our old boat after 550ish hours they were knackered, probably near useless, blades snapped off and bent.

Our shafts were dropped back about 40mm, by spacers, to fit the cutters. I dont think that's very significant on anything above say 30 or 35 feet?

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[mjf]

Re: log vs GPS

The point I was trying to make was that it depends on what is important to you vis a vis speed compromised by the cutters. Speed through the surrounding water (measured by log) or speed over the ground (GPS). I would be interested to know the difference in your test.


Thanks for the information in your chewed up blades. I did not realise that these were basically consumables......


Michael

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[duncan]

Re: log vs GPS

surely the only relevant measurement is speed through the water . SOG is irrelevant to hull or machinery issues hence normal practive to go cross tide or average runs 180 degree apart to get back to speed through water.

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[Deleted User YDKXO]

Re: log vs GPS

mjf, I'm having difficulty understanding your line of questioning as well. This comparison could not have been made on a speed over ground (SoG) basis because SoG depends on both speed through the water (Log speed) and tide. To do a proper comparative test on a SoG basis, you would have to be certain that the rate and direction of the tide was the same in both cases, which, of course is impossible unless you happen to do 2 runs in opposite direction in both cases and average the results

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[mjf]

Re: log vs GPS

Sorry, did not mean this to get protracted.... Agree with whats been said.

I always look at speed over the ground as this is my actual progress towards my destination. Speed through the water is useful surely only when trimming / adjusting settings

I was curious to see if the cutters altered the relationship between the two speed measurements. But I guess thinking about it; it is v. difficult to achieve like for like as the Wx will be different even if you use the same course at the same state of tide.


enough?

Michael




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[jfm]

Re: log vs GPS

mjf this test is about how ropecutters reduce your speed. If they reduce it by say 2 knots then that's 2 knots off your SOG and 2 knots off your speed thru water. It is not possible for ropecutters to affect STW and SOG differently, I mean to affect your STW and your SOG by different amounts.

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