Optimum RPM range for big blocks

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rickalan35
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Optimum RPM range for big blocks

Post by rickalan35 » Fri Jul 10, 2015 9:30 pm

I would like advice.

My new boat has big blocks ('94 Trojan 370 express). My previous boat (Tricabin) had 350's that I babied along at hull speed, sometimes at 1400 rpm and often at 1800 rpm.

I've only had our new boat out twice, mostly because I've been spending time with the shingles virus for the past ten weeks... ouchhhhhh, men! I suggest that you consider getting the shingles needle.

Regardless, I've now been out in the new boat, exactly twice. A little bit nervous the first time, checking on everything during the first run i.e. "Hope this thing doesn't sink." One hour at 1200 rpm

Last week-end we ran along at 1800 rpm. All ran well, but obviously I haven't exactly been a speed demon.

My question is this: Can you give me your three optimum rpm ranges to run 502's at, when not creeping along at idle?
1. Slower cruising
2. Medium cruising
3. A bit of a hurry.

I don't want to push these engines hard because it's my nature, I want them to last and I suppose because I'm seldom in a big hurry. These big blocks are a first for me in a boat. Thanks
Trojan 1994 370 Express, 502 Bluewaters

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prowlersfish
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Re: Optimum RPM range for big blocks

Post by prowlersfish » Fri Jul 10, 2015 11:12 pm

1. Slower cruising hull speed or less (aproxx 8 knots) any thing between hull speed and on plane is not a good place to run
2. Medium cruising apox 3000
3. A bit of a hurry. aprox 3500

you may find the boat will feel better at some rpms and that maybe the best rpm to run ( the sweet spot )


A flow scan could help a lot to find the best range to save fuel
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todd brinkerhoff
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Re: Optimum RPM range for big blocks

Post by todd brinkerhoff » Fri Jul 10, 2015 11:49 pm

You have 502's vs 454's? I think the 502's wot is in the area of 4600. I have the 454's. If I keep it at 1500 or below, there is significant improvement in fuel efficiency. Anything over that and there isn't a ton of difference from slow to cruise. I can maintain a decent cruise of about 18-20 mph at about 3000 rpms. Your fuel burn will be very high with those 502's even when compared to 454's.

You have a similar design as my 10.8 Meter Express. You have the Carver model. Your boat will feel better at the 3800-4000 range, but the fuel burn will make the oil companies smile...
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kevinz
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Re: Optimum RPM range for big blocks

Post by kevinz » Sat Jul 11, 2015 7:48 am

I run our 1995 350 express with Crusader 454's at 3200-3500 rpm. My Garmin GPS says 22-25 mph and fuel burn rate of 21-24 GPH. She is on plane with the four barrels kicked in and what a sweet sound !
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rickalan35
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Re: Optimum RPM range for big blocks

Post by rickalan35 » Sat Jul 11, 2015 10:48 am

Thanks men. Appreciated. I think I'll invest in a flow scan.

We're going out today, anchoring overnight and coming back tomorrow. It's only about three miles from our marina to a bay we like, so only a 35 minute run at 1800 rpm to get there. By the way, at 1800 rpm my GPS indicated 8.9 mph.

So, I plan to run out there at 3000 rpm's (which seems to be one common suggestion from all of you) to see how the boat performs and what it feels like.

I assume I'll put the tabs down till they appear to make the bow dig (like my former TriCabin)...... or watch the tachs in case the tabs make them drop a bit of rpm. Thoughts, please ???

To comment on tabs with the Tricabin at planing speed: I rarely cruised over 1800 rpm but a few years back we raced a thunderstorm packing water spouts (fear made thoughts of the boat secondary) and I had it at WOT (as I recall around 3400 rpm) with the tabs down. But I brought the tabs back up by approx 1/4 to find an optimum spot to run at, any lower position was making the bow appear to dig in and made steering difficult. We were on a Y-shaped lake (Upper Rideau Lake) and we ran up the left arm of the Y as the storm broke down the right side and managed to avoid it.
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captainmaniac
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Re: Optimum RPM range for big blocks

Post by captainmaniac » Sun Jul 12, 2015 12:48 am

rickalan35 wrote:By the way, at 1800 rpm my GPS indicated 8.9 mph.
Different boat, but engine speed stats will still be pretty similar.

You are probably wasting a lot of fuel and time at 1800RPM.... At 1500RPM you will probably be doing 90% (8 knots) of that speed at 75-80% of that fuel consumption (cheaper). To find the most efficient 'hull speed', if your bow starts to rise, you are going too fast. Increase your engine speed from idle until the bow starts to rise. Then back it off to just before the bow rise. That is your hull speed, and most efficient cruising speed.

Or depend on the old school formula : hull speed = 1.3 * sqrt ( waterline length).

Regarding tabs - it's a science. There is no clear pre-defined tab position. Their use is a science - it is like ailerons on panes - the position they need to be in depends on a lot of factors.

FULL DOWN is normally only needed when you are just crawling on plane - probably at or around 2000RPM.

As the boat accelerates, less tab is needed. If you tap up on the tabs, you can feel the boat 'free up' (maybe reduced RPM, faster speed). Don't mash the tab controls - give it a tap for a half second to full second, and wait 2-5 seconds to see how the boat responds. Based on that, add more or less tab and wait for the boat to respond. When you are flying 'free', and at the highest speed for a given RPM, you know the tabs are set right.

You can have too much tab before you perceive bow steer (bow steer is when over tab becomes dangerous...) so suggest you take it back a notch as per what I have suggested above.

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todd brinkerhoff
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Re: Optimum RPM range for big blocks

Post by todd brinkerhoff » Sun Jul 12, 2015 7:48 am

Captain Maniac is right...Cruising at 8.9 (1800) is using a lot of unnecessary fuel. Your waterline length on that boat is about 35 feet, bringing you hull speed down to about 7. As I said, 1500 is the magical # for me. It makes a huge difference. Fuel flow meters are key as well as patience in travel time when trying to find "efficiency" with these big blocks.

Another good addition are larger trim tabs. I stepped up to 30X12 with down turned sides (forgot the technical term) and it has made a huge difference in bringing my cruise speed and bow down. The next thing would be 4 blade props.
1991 Trojan International 10.8 Meter Express hull# 003 - 454 Crusaders
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Re: Optimum RPM range for big blocks

Post by RWS » Sun Jul 12, 2015 8:56 am

here is my often posted fuel efficiency table for my 10 meter express (not mid cabin) 454 Crusaders - quadrajets - 4 blade props - larger trim tabs w/drop fins.

Done in 2003, prior to the diesel repower.

RWS

RWS_GAS_CHART.jpg
CRUSADER 454 GAS ENGINES
RWS_GAS_CHART.jpg (222.04 KiB) Viewed 2394 times
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todd brinkerhoff
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Re: Optimum RPM range for big blocks

Post by todd brinkerhoff » Sun Jul 12, 2015 9:09 am

Drop fins...that's the technical term...
1991 Trojan International 10.8 Meter Express hull# 003 - 454 Crusaders
1961 Century Raven 22 - Gray Marine 327

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rickalan35
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Re: Optimum RPM range for big blocks

Post by rickalan35 » Sun Jul 12, 2015 11:43 pm

RWS, Thanks for including the gas efficiency table.

If I'm reading the table correctly and I'm definitely not sure if I am, it would appear that 2800 rpm is a good spot to run at, if up on plane. Please tell me if I'm reading the figures correctly.

During the past week-end, I took my '94 up to 3000 and ran along for a while. As i did, I eased the tabs down and watched the bow pulpit drop till it felt like an optimum position. The lake was fairly busy and I slowed a couple of times for some small craft.

Then I ran at 2800 for a mile or so, at which point I noticed 19 mph on the chart plotter, which I believe is pretty close to the figures in your table. The boat felt pretty good at this speed.

I plan to run at 3500 sometime later this week, whenever I have time to go back to the marina.

I've got the stock tabs now, of course, but I plan on checking out drop fins as an upgrade.

Right now though, it sort of looks like I'm going to be around 1500 for the best fuel economy and 2800 for cruising speed when overall time is not really an issue.

Thanks to all for the comments and ideas. Much appreciated.

Rick
Trojan 1994 370 Express, 502 Bluewaters

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RWS
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Re: Optimum RPM range for big blocks

Post by RWS » Mon Jul 13, 2015 6:03 am

you are reading the chart correctly.

of course, that chart is for a different vessel


select a throttle/rpm position.

adjust the tabs while watching the GPS ground speed

then dial in the tabs for the best speed at that RPM

different conditions will provide varying results as load, fuel weight, wind, tides, current and chop will all combine to create variables for the running conditions

ALWAYS watch the GPS for speed when adjusting the tabs

RWS
1983 Trojan International 10 Meter
Twin Yanmar 315 Turbodiesels
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rickalan35
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Re: Optimum RPM range for big blocks

Post by rickalan35 » Mon Jul 13, 2015 10:10 am

Okay, RWS will do. Many thanks.

Rick
Trojan 1994 370 Express, 502 Bluewaters

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todd brinkerhoff
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Re: Optimum RPM range for big blocks

Post by todd brinkerhoff » Mon Jul 13, 2015 11:19 am

Hey Rick,

Would you be able to do me a big favor??? I am looking for some info on your particular vessel. Would you be able to take a look at your stringer system and maybe take some photo's. The Trojan 370 is derived from the 10.8 Express, but Carver changed the stinger system and modified the hull. Would you be able to tell me how your stringers are set up, whether they are full fiberglass, hollow, laminated plywood, etc. I am trying to create a little history between your boat and mine.

Feel free to call me also. I will pm you my #.

Thanks,
Todd
1991 Trojan International 10.8 Meter Express hull# 003 - 454 Crusaders
1961 Century Raven 22 - Gray Marine 327

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captainmaniac
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Re: Optimum RPM range for big blocks

Post by captainmaniac » Mon Jul 13, 2015 12:56 pm

RWS wrote:ALWAYS watch the GPS for speed when adjusting the tabsRWS
And remember that it will take the boat a few seconds to respond to changes in tab position. Give the tab control a tap or two and wait a few seconds to see what the effect is.

rickalan35
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Re: Optimum RPM range for big blocks

Post by rickalan35 » Mon Jul 13, 2015 1:08 pm

Hi Captain,

Thanks for the advice. Will do.

I will also be attempting to make up a speed/rpm/tabs/ table of my own for various rpm levels.

For now, I think I can vouch for: 2 passengers, 1/4 fuel, no wind, no wave action.

2700 rpms/ tabs 3/4 and GPS speed of 19 mph

1700 rpm/ no tabs / and GPS speed of 8.9 mph
Trojan 1994 370 Express, 502 Bluewaters

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