RPM vs speed with 75HP engines

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RPM vs speed with 75HP engines

Post by deising »

Well, after more grueling repairs and upgrades than I expected to make, we are ready to shove off for the Great Loop/Down East Circle this Fri.

I have only had the opportunity to make a handful of short sea trials in between engine repairs, and I am learning first-hand that as you increase RPM, there is quite a 'dead spot' where your speed does not increase all that much for all the extra power added. To be accurate, I should note that Dick Tuschick also pointed that out during our half-day underway tutoring session.

In these later trials, we already have the majority of weight on the boat, except for the dinghy, and she is almost even at the waterline at the bow with the stern about 5 inches high. That seems excessive, but it is amazing how much the bow seems to rise when underway at 12 or more kts.

So far, it seems that 3200 is a nice cruising power setting and I see about 12-13 kts with that.

These are just initial observations. Does anyone else with the 75HP engines have any comments or advice?
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Re: RPM vs speed with 75HP engines

Post by SecondWind »

Duane, If you are nose heavy, which your description says, you have a great chance of submarining (stuffing a wave) a catamaran. Sounds like you need some weight redistribution so that you are reasonably level at rest. The bow will tend to rise in power boat as power is applied, but you need to keep your weight centralized not in the bows or sterns. Things that you can get away with in a monohull don't work well in catamarans.
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Re: RPM vs speed with 75HP engines

Post by duetto »

hi duane,

you are quite right as to the "wall". we describe duetto as a 2 speed boat, either 7 knts or 12+. your 12-13 @ 3200 is consistent with duetto. i assume you have 16x13 props.

as to the bow down, duetto also rides with stern up slightly at rest. once up on plane it reverses with the bow up slightly. we've never had a problem with handling.
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Re: RPM vs speed with 75HP engines

Post by deising »

Well, Terry, I understand what you are saying, but our bow is 5 feet above the waterline, certainly much higher than the sailing cat, so hopefully that will help prevent stuffing a wave.

There is practically no place to stow large, heavy items except for the bow. I already have all the canned goods and heavy spare parts that will fit under the floorboards midships, and lots of heavy liquids and tools midships under the settee. My scuba tank, spare anchors, bicycles, etc, are all in the large forward locker.

John, that is good to read that what we are experiencing is quite normal for that boat and that it has not proved a problem for you.

Thanks to you both. Other comments welcome.
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Re: RPM vs speed with 75HP engines

Post by SecondWind »

Duane, The issue is not one of the height of the bow. It is an issue of buoyancy and has nothing to do with whether it is sail or powercat. Perhaps you just have too much stuff onboard. Be careful out there and have a safe trip.
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Re: RPM vs speed with 75HP engines

Post by deising »

Thanks, Terry. I understand that in an ideal world we concentrate the weight amidships and leave the bow and stern light, but boat design doesn't often let that happen. The dinghy davits are as far aft as you can go, and as I said before the only place to stow large objects is in the bow lockers. In those lockers, I have 50 lbs of bicycles, about 60 pounds of spare anchor/chain/rope, a 30 lb SCUBA tank, 30 pounds of spare anchor rode, and a few light items. The primary anchor is 55 pounds and near the bow, but the rode is closer to midships. I can't see any of that stuff as being too much or unneeded.

BTW, I hoisted the dinghy this morning and see that the waterline is now even fore and aft at about 3 inches above the actual water level.

We will certainly try to have a safe and enjoyable trip. Thanks!
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Re: RPM vs speed with 75HP engines

Post by jgonnerman »

I realize that Divi Di is underway but I just could not restrain myself from adding to this thread. As a lapsed naval architect with some exposure to planning hull design (my supervisor in those days was D.L. Blount who now has his own firm) I can add some items to the discussion.

The speed power curve's for all planning hull's is a non linear transition curve when changing from a displacement to planning mode. Power required to achieving a full plane is 20-30% greater than the power required to remain on a plane at minimum planning speed for a given hull from. A complicating factor for a PDQ is in the displacement mode at some point the hull speed is achieved with a concurrent added drag of bow wave interference. The classic hull speed calculation of 1.34* the square root of LWL would be about 7.81 although very fine catamaran hulls (L/B) defy this simple calculation. I don't think the PDQ hull fits into the fine hull form class. On the PDQ from my observations hanging over the bow this hull speed is over the speed at which the bow waves tend to interfere which would suggest that a slightly slower speed would be a bit more fuel efficient. The boats displacement also has impact on bow wave interference.

Firstly weight is a death sentence for multihulls and planning hulls. Duane's reference line of the waterline is "soft" as on my boat the waterline is well above the immersion line even when I have full fuel and water so it depends on where the boat yard applied the masking tape. I think a better reference is the spray rails.

In any event considering that the Great Loop trip in not too far offshore and there is access to West Marine, Walmart and Amazon's drones it sounds like Divi Di is way overloaded. Of course one work around is reducing fuel load and water.

In any event this is good year to make the trip given the price of diesel it just will need to burn more as that is the primary difference in the performance of a power cat.

As far as the notion of a PDQ powercat having a potential threat of pitchpoleing I would not want to be in those weather conditions.
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Re: RPM vs speed with 75HP engines

Post by deising »

Thanks very much, John, for all that detail.

I just looked at the spray rails and they are 2 inches above the immersion line at rest. The stern has the waterline painted up to the rubber cap over the curved portion of the transom, and the immersion line is about 3 inches below that. So, at rest we trim down slightly at the bow.

As for weight overload, I definitely see the point you and Terry have made, so we plan to use up canned goods and beverages and only stow about half of what we started with.

This might seem elementary for you with power cat experience, but I am finding that in the current state, I need to go full power to get above say 13 kts, then I back down to 3200 RPM and the speed in no current, minimal wind settles in about 11-12 kts. If anything strong headwinds or wakes knock the speed down, we just keep slowing down with the bow pitching up and the wake getting squarish. I then need to go back to full power to get back "on plane." Since this was not the case when we brought the boat back essentially empty from Stuart, I guess weight is pretty much the answer.

I don't like being that close to the edge all the time and don't want to run the engines any harder. I guess the 100HP might have been a better choice, after all.
Duane Ising
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Re: RPM vs speed with 75HP engines

Post by duetto »

hi duane,

you're observations about the 75hps are correct. if you try to run under full load in anything but flat seas you'll continually drop off plane, accelerate, back down, repeat. after 10 years we've found just run it at 3300-3400 and take a qualude :). it is what it is. today we ran for 6 hours at 3400 rpm with no problems. tomorrow we'll run 9 hrs at the same speed.

as far as bow burying, last year returning from eluthera we ran downwind in 6-8' waves all day. when we got off hopetown the biggest waves were more than 8'. we never came close to burying the bows even doing 18-20 knots down wave faces. this is one example. after 10 years we've seen these conditions before.
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Re: RPM vs speed with 75HP engines

Post by deising »

Thanks, John. Guess I should have done more homework before this purchase.

As for just running at 3300/3400, right now I can only get 3500/3600 out of the port and stbd engines at full power (as opposed to 3700/3800 during the sea trails when the boat was very light), so I am not comfortable running so close to that.

Another unfortunate problem that I need to address is the port engine climbs above its typical 175F on the temp gauge once I try to run above 3200 RPM. For now, it seems I am a slow trawler unless we are in flat water.
Duane Ising
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Re: RPM vs speed with 75HP engines

Post by jgonnerman »

The forum is filled with the overheating issue and having much lower water injection temperatures in the cool/cold Pacific I used to think that at least there was some benefit living in So CA. Last summer ran the engine to full power after entering the harbor entrance and off went the port engine temp. Did what all the forum's remedies said to do which is clean the heat exchanger bundle, pulled them went to local radiator shop which does a lot of marine work replaced and problem solved. Pulling the bundles was really easy and quick. In my case it is easier than an oil change or raw water impeller replacement.

As far as boat speed when loaded if I run at 2800+ I get 9.5+ boat speed which is faster than my J-120 under power and faster than a GB 36 and all it means is a bit more diesel and I don't worry about melting the engines. I suspect that once you burn off about a third of your fuel load about 500 pounds then your back on a plane. I find that my boat will stay in a planning mode at about that load level.

Also if you have a watermaker dump half the water. Just a thought
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Re: RPM vs speed with 75HP engines

Post by duetto »

i agree with jon.
we had the same symptoms a few years back. checked raw water entering and exiting heat exchanger with heat gun. the temps were virtually the same. we sent the heat exchanger core to a local radiator shop. they removed the aluminum shield and found black goo packed between tubes. cleaned out and put back together runs at 185 all day long.
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Re: RPM vs speed with 75HP engines

Post by deising »

Long overdue for a report; sorry. We started our Loop/Down East Circle cruise on 28 Mar from SW FL and crossed the Okeechobee waterway. We stopped at Stuart Yacht for 1.5 days and had two things looked at, with the port engine cooling problem being the only one solved, unfortunately.

They ran Barnacle Buster through both the raw water and coolant circuits and there was a good bit of sludge that came out. The cooling problem has not manifested itself since, so I consider that solved.

I was not aware that other owners have reported you cannot fill the engine oil level to the top dipstick mark or it will simply 'blow out' in short order. I was checking the oil every other day and adding a quart, and quite dismayed to see it down a quart the next time. I am now considering the halfway mark as full and the stbd engine seems to stabilize there, but the port engine still drops a bit faster than it should.
Duane Ising
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Re: RPM vs speed with 75HP engines

Post by duetto »

hi duane,

first let me say that we have always been able to fill to the "full" mark on the dipstick and not have it blow out.

when you check the oil are you removing the dipstick, wiping it and reinserting?
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Re: RPM vs speed with 75HP engines

Post by deising »

Hi, John. Yes, I am wiping the stick and slowly reinserting it fully.

Our engines were installed in 2006 so just maybe Yanmar made a change to the markings on the dipstick since your model year. I got that info from Dick Tuschick, by the way. Another owner with my same year stated he has the same issue, so he just considers the middle between the marks to be "full" and it works for him.
Duane Ising
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2006 PDQ MV 34 - hull 91, 75HP, 3-blade
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