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Old 08-29-2014, 02:26 PM   #41
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Cappy. I totally agree. We have the 4-18 Perkins with a 2.9-1 gear. She has the factory prop 18x12. At 2600-2750 rpm, we do 6.2- 6'5 and burn .75 gal per hour. At 3000 rpm, we hit 7.5 and burn 2.6 per hr. At 2600, very quiet,smooth,no smoke no strain, every one is happy even the dog.


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Old 08-29-2014, 04:04 PM   #42
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Personally, unless life or limb depended on it, my engine speed of 2800 max won't ever be used. I try to never go above 2000. This gives me around 7.3 K. That's good enough for me. I am quite content to laze around at 6.6 at around 1650.
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Old 08-29-2014, 04:47 PM   #43
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One should run an engine at rated rpm for a minute or so every now and then or you won't know if you're engine is 100% alright. My old Perkins ran perfectly till I took it to 3000rpm (rated) for at least a minute and then it quit. Something was wrong and then I knew. Otherwise I would have been proceeding with a much greater chance of engine failure as time went on.

I'm rebuilding my fuel system now and the first thing (after another oil change) after two years of sitting will be to run steady for 3 or 4 minutes at WOT (3000rpm). Then I can go fwd w confidence.
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Old 08-29-2014, 04:53 PM   #44
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The Perkins 4-108 is rated at 4000 so we are in the same range. I have never been over 3000 and then only for a small amount. Doesn't seem to make a big difference. This hull is not going go fast.

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Old 08-29-2014, 07:00 PM   #45
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Sortie,
They rate the 4-107 and 108 at 3000, 3600 and 4000rpm. The latter for taxi cabs in England. I don't understand the different ratings. Perhaps it's w different cams, valves, ports, fuel injection, manifolding, compression and many other variables. All the marine applications (including gen sets) are rated at 3000rpm. I don't think the higher speed ratings have anything to do w boat power and I only pay attention to 3000rpm as a rating. It's nice to know an engine can give good service at much higher speeds though.
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Old 08-29-2014, 07:42 PM   #46
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Eric, I think your description is right on. I find the engine is at its best at 2600, I can not really understand the ranges in the engines. Personally, I think the best engine for the hull was the 4-236. Having said that, I am not concerned enough to engage in an engine swap for another Knott of speed. The 6.2 is great and the economy even better.

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Old 08-29-2014, 07:53 PM   #47
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John,
You usually run at 2600?.

Is the "4-18" the 236 cu in engine (Perkins)?

96% of the time I'm at 23 but at times I run at 2500. A bit more noise there. I need an intake silencer. One of my many projects on the board.
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Old 08-29-2014, 08:28 PM   #48
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Eric, I have the 4-108 50 hp . My mechanic says it is a 400rpm, but that's a bench number. He says it is a 3600rpm. This engine gets noisy after 2800. Between 2500 and 2800 is pretty smooth.
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Old 08-29-2014, 08:56 PM   #49
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The Campion version of the Prairie has a Merc 120hp Gas engine in it. They STILL only do 8 or 9 knots MAX. Even the high torque, high rev gas engines can't make the hull move any faster!

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Old 08-29-2014, 08:59 PM   #50
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The original 4-108 Perkins 50 HP diesels in the fist 12 KK Manatees were also 3600 RPM. According to the owners, the boat would do 7.5 knots, but was happy at 6.5. There's still a few of them out there with the Perkins, but all the re-power jobs I've seen have gone with other higher HP Volvos.
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Old 08-29-2014, 09:46 PM   #51
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Very interesting thread. It is apparent that the bottom design of the Prairie29 while similar is different enough from our Marben27. I would venture with Eric’s approval, to say the Marben has a more FD bottom than the Prairie but both will be sighted as SD. Marben was so designated by the builder, although I wonder if that was not a selling point in 1978 during a fuel crunch period to imply a bit more speed than actual.

The water line formula of length x breath has this hull at 6.8 hull speed. Our boat has the dreaded rear seal leaker Perkins 4-154 which is rated at 58 HP at 3000 RPM. We drive through a 3:1 reduction gear throwing as 22 X 16 LH three bladed wheel. At 2400RPM the shaft is 800 turns and we average 6.6 knots burning 2 gallons per hour. At the 2200 RPM range we average 5.7 Knots and burn 1.75 gallons per hour. At 2150 we average 5.4 knots burning 1.55 gallons per hour.

I marvel at the claims of averaging 1 gallon per hour or less on either the Perkins 4-107 Note and the fuel burn of similar claims on the 4-236 at relative RPMs. I don’t dispute the data, just marvel as I am not able to match it in use.

Our last boat has a 70 HP Hercules and it burned 1.75 at 1800 with a 2: 1 gear throwing a wheel that I have forgotten the measurements but seemed it was overproped as it was an actual small harbor tug at one time.
The Hercules was rated at 2600 RPM. Another example of a similar size engine burning more that many claims.

All that aside, Having acquired a Perkins 4-236 engine with the intent of future plan of installing with a 2:1 reduction that will allow a 1800 RPM giving 900 RPM at the shaft (2600 RPM rated) which if 6.6 knots currently being acquired at 2400 RPM (800 turns at the shaft) with a proper Vicprop formula wheel should give me in excess of the water line formula outcome. I would suspect 7 knots plus and be disappointed were it less.

Let me be clear, were I not to do any change and remain with the Perkins 4-154 turning 2400 and 2 gallons per hour fuel burn, life on the water will continue to be desirable, affordable, and project free. All have merit. For the sake of the discussion the above is offered as comparable or questionable information seeking comments pro or con.
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Old 09-03-2014, 10:31 PM   #52
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I finished my delivery trip 936 miles, 240 gl diesel (3.9 MPG). I never went over 2000 rpms. Now were doing putting around Cape Cod. I have never gone over about 1750 rpms here at home ( I am a slowpoke!)

My fuel consumption is in the viscinity of 1 gph at 6 knots. I am using one tank now, to totally empty the other to keep it clean (Now that they are all cleaned out) and emptying the other, as we wind down the season, to strip it of schmeg also.

In the process of doing this, I was sucking from one tank, and returning to the other. before each trip, carefully measuring and recording fuel tank levels.

One one 5 hour (of running time) trip I burned 5 gallons, and never exceeded 6.5 knots (Usually 6.0 or 6.3) So the gallon an hour figure is pretty reliable. However, on our delivery trip, trying to get up to 7.3, 7.5 the consumption was higher (as noted by the average consumption)

I suppose, if I kept it down to 1000 I would burn the lowball number of .75 per hour. But, with the Grandkids aboard, I don't have THAT much patience!
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Old 09-03-2014, 11:59 PM   #53
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How do you determine your speed cappy 208?

Sortie I think the 108 should be propped to 3000 rpm. And I've read specs that put the 107/108 at 36hp @ 3000rpm, 42hp at 3600 rpm and 50hp @ 4000 rpm. You are fortunate you've got the normal engine w/o sleeved cylinders. Will never leak. I would prop and run the little Perky at/to 3000 rpm. This is the way Willard had it set up when new but they actually over propped it 250 rpm so a Willard 30 only made about 33hp as delivered. As soon as if discovered that I took an inch out of the pitch on my prop and the engine seemed to free up and sing a relaxed tune ... At 3000rpm. Been topping out at 3000 ever since. Not exactly of course but as close as I can. If I can be that subjective the engine seems most happy propped at 3050 rpm.

My new engine is a Mitsubishi with the same bore and stroke as the Perkins. I run Willy at 6.15 knots burning 1 gph at 2300 rpm. Seven knots is flat out but a FD boat takes more power to run at hull speed and less to run a knot below. I think I would burn 2 gph at hull speed. And almost all FD boaters will run a knot (or at least 1/2 of a knot) below hull speed for this reason. The Willard is a FD boat w a WLL of 27.5'. For the FD boat 3 or 4 hp per ton is about perfect for total power. Most SD boats have 7hp per ton and up but then they require more power. I calculated my Willy to be just a tad over powered and sure enough she has 5hp per ton. Over the years I've determined 3hp per ton for the right boat and skipper will be plenty but 4 fits most every situation for the FD hull. Only about 15hp is needed to cruise Willy and that equates to about one gallon per hour consumption. Many Willard owners have collaborated that on the yahoo group site. On our trip to Alaska 1000 miles at least (850 as the crow goes) so we had a fairly accurate account of fuel burn.
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Old 09-04-2014, 07:12 AM   #54
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Speed by GPS at slack current. One thing I have determined: go fair current whenever possible! It is quite embarrassing when a lobster Pot buoy passes me!
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Old 09-04-2014, 10:05 AM   #55
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Here's the bottom of the Prairie 36, which is the 29's big sister. Also Hargrave designed. Shown here after soda-blasting and before the barrier coat and bottom paint was re-applied.
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Old 09-04-2014, 10:05 AM   #56
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Cappy,
I don't know if there is any "slack" sea water in coastal areas. This fall I'm going into Lake Washington (behind Seattle) where they have a measured mile marked off alongside a floating bridge. I want to make a rpm/speed chart over the measured mile. I'll use the measured mile for 2000rpm to 2500rpm. Then I'll make some other recordings re other speeds .. including top speed.

Part of me wonders why because I'll be applying the results in tidal waters ????

Yes crab/lobster floats can be bothersome.
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Old 09-04-2014, 10:13 AM   #57
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Thanks Tom that's a fairly good picture of the bottom of the boat. Is the 36 real close in hull form to the 29? I've been wanting to see a better pic of the hull of the Prairie for some time. A soft chine but harder than most soft chines, a long straight run aft w what appears to be a fair imitation of an ice breaker bow. No deep forefoot for this gal.
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Old 09-04-2014, 10:34 AM   #58
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There's slack water every 6 hours and 15 minutes (at least in my diurnal world) Sure, there is fair, foul, and slack. After hours of running, it all averages out. Even on short trips one can discern the current, and the velocity. To be honest, I have NOT run reciprocal courses to completely eliminate the current. But being fairly well experienced in commercial vessel operations, I am somewhat confident in my ability to cipher the dilemma.

It's not perfect. the current changes early, late, or on time. But for my speed calculations (taking into account the tide current predictions) I am fairly confident in the speeds.

For me, its about the hours run, and the fuel consumed while running. More can be told by fuel burned, and how much is burned per hour than how many miles have been run. Most people when asked about fuel consumption per hour are completely clueless. (not being snarky, just observant)
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Old 09-04-2014, 10:43 AM   #59
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Quote:
Originally Posted by manyboats View Post
Cappy,
I don't know if there is any "slack" sea water in coastal areas. This fall I'm going into Lake Washington (behind Seattle) where they have a measured mile marked off alongside a floating bridge. I want to make a rpm/speed chart over the measured mile. I'll use the measured mile for 2000rpm to 2500rpm. Then I'll make some other recordings re other speeds .. including top speed.

Part of me wonders why because I'll be applying the results in tidal waters ????

Yes crab/lobster floats can be bothersome.
One of the questions I have to ask when at work often is: what is your slack water speed? I often ask, because in the process of proceeding up or down a channel, it is necessary to discuss overtaking (or to be overtaken) of another vessel. I know (for instance) that my tug (and barge) make 9.4 knots when loaded, and 10.4 knots when empty. So, if I ask another vessel what speed he makes in slack water, I can readily determine if it is even possible for me to overtake, or make it in time for a next bend in the channel, or to allow for other traffic. But way too often, the response I get is: "Well, I'm doing 8.5 with the tide" That wasn't the question. Because, I know we have fair at the present time, and the channel has varying current (based upon stricture, and confines of the channel) that will change over the mile or two it will take me to overtake him. I have to know what the slack water speed is, to make informed decisions of both vessels situation. This is particularly appropriate when two vessels are meeting at the confluence of two separate rivers, with differing currents effecting each.
This brings up an interesting topic. The measured mile is only effective if you run it one way, then turn around and run it the other way. This way you have one leg with foul current, the other leg with fair. (assuming this was not at slack water.

It is not important to know the speed over the bottom, unless you have erased the current effect. However, it IS important to know your speed through the water, when applying set and drift in voyage planning.

The other way to know current (other than by visually seeing it ((Then estimating) on buoys and fixed structure) is to use a current book, and extrapolate the velocity at the time you are running.

On the East coast we have an excellent reference book that pretty much has all the current velocity at each hour of the day already printed and ready to use. (Eldridge Tide and Current Book) They are worth their weight in GOLD. In addition to about 100 pages of extremely helpful seamanship hints (customized to the local waters) they have tide and current predictions from Maine to Miami. All in easily usable format for the boater. We even use them in the coastwise tug industry.
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Old 09-04-2014, 11:06 AM   #60
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Yes of course I do plan to run both ways ... elementary as you imply.

I've never been able to get consistent results w the GPS in sea water. Turn around and go the other direction and the speed is different or it will be in a moment. And measuring over a long distance obviously won't be stable so it's off to the lake for me.
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