Looking for some insight/guidance on what we can expect under motor (acknowledging the variability of conditions). Generally speaking, we don't do a lot of motoring. We're on our first extended cruise and have run into unfavorable winds. I have a two-blade prop (not sure of the pitch) and have been maxing out at 4-4.3 knots. RPMs are at 2500, temperature a steady 160. We've got a long slog tomorrow with 8-10 mph winds forecast to be on the nose. Wondering if I just need to accept the fact that this is about as good as I'm going to get from the 25XP with my current prop configuration. Would also welcome thoughts on what I could expect by investing in a new prop.
Thanks in advance.
M25xp with a 3 blade Michigan Sailor 5.5-6 depending on how heavy we are and wave conditions.
M25XP 6.5 -7knots with a 3 blade 10 pitch prop. in protected water with no wind at 2500rpm.
M25 6.5 -7 knots with a 3 blade 15 x 10 pitch prop. in flat water with no/little wind at 2800rpm, clean bottom, 2500 rpm is 6+ knots. WOT that I check regularly both in gear and out of gear is 3000.
My PO removed the two blade, so I don't know what that performance was nor do I recall the dia / pitch - it's somewhere at the bottom of my locker.
As I recall the factory spec. 2 bladed prop is a 15" X 10".
Thanks for the input. Obviously I'll be shopping for a 3 blade this winter. For the short term, any thoughts on running at 2800? I've pushed it on a few occasions, but haven't run that high for any significant time.
Two questions - is your rpm accurate? Is your speed over ground accurate?
Yes on both fronts.
Quote from: Stu Jackson on August 03, 2019, 11:08:05 AM
M25 6.5 -7 knots in flat water....with no/little wind at 2800rpm, clean bottom, 2500 rpm is 6+ knots. WOT that I check regularly both in gear and out of gear is 3000.
I get approx same speed as Stu, on my 25XP but I have a 3-blade Flexofold—which I "hope" gives me more speed under sail than a fixed-bladed prop.
Less drag.
Quote from: sailscituate on August 03, 2019, 01:39:15 PM
Thanks for the input. Obviously I'll be shopping for a 3 blade this winter. For the short term, any thoughts on running at 2800? I've pushed it on a few occasions, but haven't run that high for any significant time.
I ran at 2800 from SF to Maple Bay, British Columbia, in 2016, 1500 nm, 7.7 knots. Don't "baby" the engine. Still runnin'. :D
I put a 3 inch HX on eons ago.
180F
All of our racers down play the 3 bladed prop because of the drag, but it will back down right now. Wife does all of the motoring in and out of the dock. Some times I get a little worried if she comes in a little hot, but in 25 years she has always stopped well short of that steel sea wall. Fixed or folding, your choice. To me, I'll stay with 3 blades. (i don't race)
Hey Guys,
would you go back and edit your posts in this thread and include the following:
Boat Name
#of Prop Blades
Max Engine RPM
Typical Temp at Max Engine RPM
Avg Speed at Max Engine RPM through the water.
I'm compiling a spreadsheet with averages that might help everyone.
Thanks.
Eximius:
3Blades
2100 RPM
152ºF (148ºF Thermostat)
6.5knots
Thanks.
Paul
Guys : A while back I wrote a Mainsheet C34 Tech Note article on "Boat Speed; Prop and Engine"!! You might find it interesting reading!!
A thought
Quote from: Noah on August 03, 2019, 05:56:15 PM
Less drag.
LOL I meant he's measured speed over ground, how? sorry.
Quote from: KWKloeber on August 03, 2019, 05:53:43 PM
Quote from: sailscituate on August 03, 2019, 05:30:55 PM
Yes on both fronts.
speed how?
Ken has an important question. If you're using GPS speed, but running against the tide your speed through the water may be in the proper range. If you're using your knot/log how do you know it's accurate? One way to use GPS to determine speed through the water and to check you speed in both directions and apply a little math...
Hi SS,
If you are considering a new prop, I suggest checking out a folding prop. I got more motoring speed as well as sailing speed.
Also, as others are indicating, be sure to check you actual tach rpms with a laser device.
Regard,
Ed
Quote from: Ted Pounds on August 04, 2019, 05:16:56 PM
One way to use GPS to determine speed through the water and to check you speed in both directions and apply a little math...
Another is to use it to travel a known reasonably long distance, and apply a watch to calculatate average speed. Bflo harbor breakwall has a marked measured mile, but if you use gps you're "off" the degraded amount only at the start and end so (just for a hypothetical example) say 30' over a mile or two or five. If you use gps speed, it can be off the degraded amount (in this hypothetical again, 15' in any compass direction) on each and every reading it reports back. If you are powerboating at 20 kts that's not significant, but......
Guys : Look on the NAOA (sp?) charts in your location - there are numerous "measured miles" noted.
A thought
A good rule of thumb is to determine your max RPM and cruise at 80%. So Floor It, then back off 20%.
Quote from: KWKloeber on August 04, 2019, 07:08:09 PM
Quote from: Ted Pounds on August 04, 2019, 05:16:56 PM
One way to use GPS to determine speed through the water and to check you speed in both directions and apply a little math...
Another is to use it to travel a known reasonably long distance, and apply a watch to calculatate average speed. Bflo harbor breakwall has a marked measured mile, but if you use gps you're "off" the degraded amount only at the start and end so (just for a hypothetical example) say 30' over a mile or two or five. If you use gps speed, it can be off the degraded amount (in this hypothetical again, 15' in any compass direction) on each and every reading it reports back. If you are powerboating at 20 kts that's not significant, but......
Ken,
I think you're missing my point. What he needs to determine is speed through water not speed over ground. The engine might be pushing the boat at 6 knots through the water, but if there's a contrary current of 2 knots then GPS speed will only be 4 knots. And the same is true using a measured mile.
I ass/u/med he was checking max speed in flat/slack conditions because he seemed sure the speed determination was correct. Surely he'd not be trying to measure if there was a current and not accounting for it. Maybe a bad assumption? :shock:
My comment was to eliminate gps error, not account for current.
my 88 25xp gets 4.6 to 5 knots in calm water, empty boat no dinghy, at 2500 rpm, with a 15/10 3 blade prop. used to get 6 knots can't figure out why I'm slower now but summer is so short here I quit looking for reasons why, seems like there is a huge disparity from one boat to the next even though the boats are the same, Stu the thought of going close to 7 knots would make me a real happy sailor, a real wheee factor, grin
Quotemy 88 25xp gets 4.6 to 5 knots in calm water, empty boat no dinghy, at 2500 rpm, with a 15/10 3 blade prop
Could it possibly be a slipping transmission? I don't know how to test for that, other than look at the transmission fluid for discoloration. Maybe a search here would give you a test to do, or someone may respond.
Jim
Maybe I can weigh in on this since now I can say I have owned an 89' with the 25xp and a 2001 with the M35BC.
With the 25xp, 3 blade fixed prop (pitch unknown), 2500 rpm, fresh bottom, 5.2-5.5 kts. By end of season I would only get about 4.8kts. Calm waters, no tide.
With the M35BC, 2 blade, 2500 rpm, VC-17 bottom paint, 6.5-7 kts. Nearing end of season, nearly no difference. Calm waters, no tide.
I can say without a doubt the 25xp struggled as soon as there was any chop, wind, dirty bottom, etc. The M35BC has the extra power that seems to push you through the chop and wind with ease and remain at cruising speed.
I've read for years about people with the 25xp motoring at 7+ kts. Good for them, I have never even come close.
As they say "the devil is in the details". The last example is not a controlled experiment: different engines two different boats/hulls and, unknown calibration of instruments.
My 1990 sugar scoop stern boat, under power, with a 25XP, a 3-blade 15x10 Flex-o-Fold prop, a "calibrated" knot meter (checked against GPS and on measured mile), will do 7 knots in smooth water, at 2,800 RPM, (NOT calibrated tach) slack tide—with the sugar scoop stern buried. I have a hard bottom paint that is diver-cleaned monthly and a standard, lightly loaded boat. Just another anecdotal example.
25xp, Max-Prop 15 x 9.3 Calibrated 2500 rpm. 5.5 knots by gps. Flat water, no wind.
I'd give a testicle for 7 Knts.