Greeting collective mind of the internet. I have decided to just keep the 454's in my boat and work on doing other items that can increase fuel efficiency and one of the items is proper props. For reference, I have a 1989 Luhrs 400. It gets up to 4500 rpm at WOT (which I never run) and it has 2 23in dia 26in pitch 3 blade props. I have decided on going to 4 or 5 blade props, but am not sure what size and or pitch. My general cruising speed is hull speed so 8.7 knots and it takes 2,000 RPM to get there. Any suggestions?
Prop size and type
- Thread starter Blarg21
- Start date
The friendliest place on the web for anyone who enjoys boating.
If you have answers, please help by responding to the unanswered posts.
If you have answers, please help by responding to the unanswered posts.
rslifkin
Guru
- Joined
- Aug 20, 2019
- Messages
- 7,583
- Location
- USA
- Vessel Name
- Hour Glass
- Vessel Make
- Chris Craft 381 Catalina
Assuming yours are the basic, low output 454s (rated 340hp for Mercruisers, 350 for Crusader), you want to end up around 4200 - 4400 at WOT. 4500 is a bit high. So keeping your current 23x26 size but going to a modern 4 blade design is likely in the ballpark. Might need to drop to 23x25, but it wouldn't hurt to talk to a prop shop to confirm.
If you want efficiency at slow cruise, 8.7 kts in a 40 foot boat is a terrible speed to run. Slow down to 6.5 - 7 kts (1300 rpm in calm water on my boat with 454s, 2.57:1 reductions and 22x25 props). You'll burn a whole lot less fuel.
Beyond prop changes, consider an ignition system upgrade. More ignition timing at low rpm under light load will help fuel consumption at slow cruise, as will keeping the idle mixture set a bit on the lean side.
If you want efficiency at slow cruise, 8.7 kts in a 40 foot boat is a terrible speed to run. Slow down to 6.5 - 7 kts (1300 rpm in calm water on my boat with 454s, 2.57:1 reductions and 22x25 props). You'll burn a whole lot less fuel.
Beyond prop changes, consider an ignition system upgrade. More ignition timing at low rpm under light load will help fuel consumption at slow cruise, as will keeping the idle mixture set a bit on the lean side.
rslifkin
Guru
- Joined
- Aug 20, 2019
- Messages
- 7,583
- Location
- USA
- Vessel Name
- Hour Glass
- Vessel Make
- Chris Craft 381 Catalina
Up until you get on plane where the rules change a bit, drag increases significantly as you increase speed. Up to somewhere around 1.1 times the square root of your waterline length, it's pretty minor. Once you get above 1.2, it increases significantly up to hull speed, then increases even faster beyond that (then the relationship changes once you get over the hump and on plane). 1kt below hull speed is another reasonable estimate for an efficient slow cruise.
Keep in mind, your 40 foot boat likely has a waterline of somewhat less than 40 feet. If your waterline is 36 feet (which is probably in the ballpark, my 38 footer is 33.5 feet LWL), your hull speed is 8.04 kts. So 7 kts is likely to be a good slow cruise. You can go faster, but fuel consumption will start to increase significantly (as will wake production, meaning you'll end up slowing down more often if you're in an area where that matters).
You can identify the sweet spot somewhat by varying your speed and paying attention to rpm vs speed as well as your wake. When you hit the point where another 100 rpm starts to produce a smaller gain in speed and your wake is starting to increase significantly, you're likely at the upper end of the range for an efficient slow cruise.
High Wire
Guru
Hull speed is dependent on waterline length, not overall. I doubt your Luhrs is 40' on the waterline. Lets assume 36' for simplicity. Square root of 36 is 6, times 1.34 is 8.04 kts. But theoretical Hull speed is NOT the most economical slow speed. If you use 1.1 or 1.2 instead of 1.34 you will get better economy. Let's try 1.15 to see what we get. 6.9 knots. So round up to 7.0 even for a realistic cruising speed. It's your wallet and your choice.
And additional blades is more drag and less efficiency. Stay with 3 blades in good health.
And additional blades is more drag and less efficiency. Stay with 3 blades in good health.
Last edited:
High Wire
Guru
Fuel Injection or carbs?
The last boat I had with calibrated Flo Scan fuel monitors had twin 5.7L Chev inboards with carbs. The economy I could get was about 1.2 NMPG around 1500 RPM and about 1.0 on a slow plane of 18.0 kts @3000 RPM. I tried everything except a full swap to fuel injection. The worst economy was between 2000 - 2500 RPM. Initially my boat was overpropped with the standard props. Would not come close to max recommended RPM. Removing 1.5" of pitch helped economy on plane as well as slow speed. I have performance graphs to show the difference. I'll attach them.
The last boat I had with calibrated Flo Scan fuel monitors had twin 5.7L Chev inboards with carbs. The economy I could get was about 1.2 NMPG around 1500 RPM and about 1.0 on a slow plane of 18.0 kts @3000 RPM. I tried everything except a full swap to fuel injection. The worst economy was between 2000 - 2500 RPM. Initially my boat was overpropped with the standard props. Would not come close to max recommended RPM. Removing 1.5" of pitch helped economy on plane as well as slow speed. I have performance graphs to show the difference. I'll attach them.
Last edited:
They are carbed. I wish they were fuel injected and I really with this were a diesel boat, but hey none were available when I was looking that didn't need a rebuild or something that was prohibitively expensive. Any suggestions on how to convert to fuel injected?
rslifkin
Guru
- Joined
- Aug 20, 2019
- Messages
- 7,583
- Location
- USA
- Vessel Name
- Hour Glass
- Vessel Make
- Chris Craft 381 Catalina
Making sure the carbs are in good shape and not overly rich under light load plus a better ignition advance curve will likely get you 80% of the gains of EFI (in terms of fuel burn) with less cost and work. Although an EFI conversion is certainly possible.
I'm pretty surprised that High Wire couldn't do better than 1.2 nmpg at low speed with small blocks. Usually those do significantly better than big blocks. And my fairly heavy boat with big blocks has no trouble getting 1.2 - 1.3 nmpg at 1300 rpm in fairly calm water. Up on plane it's a whole lot worse though (around 0.55 nmpg).
I'm pretty surprised that High Wire couldn't do better than 1.2 nmpg at low speed with small blocks. Usually those do significantly better than big blocks. And my fairly heavy boat with big blocks has no trouble getting 1.2 - 1.3 nmpg at 1300 rpm in fairly calm water. Up on plane it's a whole lot worse though (around 0.55 nmpg).
High Wire
Guru
FI MIGHT, no guarantees, get you 10% less GPH at the same RPM for many thousands of dollars IF you can find something that will work from the hot rod world. Second best choice: Repower with new FI gassers, or repower with reman Cummins 6BTs. Again many thousands of dollars.
The cheapest choice is to live with what you have until new engine time.
lwarden
Guru
Generally it's not worth it to retrofit carbed engines to FI if the engine are tuned properly. The efficiency gains are minimal. You do get better starts but what comes with that is greater complexity and more difficult troubleshooting.
Depending on the prop you have there might be some advantage to getting a modern CNC prop (I like ACME props), but from your data the props you have seem to be pretty well suited ie; not over-propped and pretty large diameter for the sized boat/engine.
If you really want a modern FI engine the best bet is to replace the current engines with bobtail units which are ready to go.
Depending on the prop you have there might be some advantage to getting a modern CNC prop (I like ACME props), but from your data the props you have seem to be pretty well suited ie; not over-propped and pretty large diameter for the sized boat/engine.
If you really want a modern FI engine the best bet is to replace the current engines with bobtail units which are ready to go.
High Wire
Guru
I was surprised too but the FloScans dashed my expectations. In the end, even though I loved the boat, the reality was it was not the boat to see me into retirement. Sold it and bought something else more affordable for several trips a week.
Last edited:
fgarriso
Guru
The boat is a semi-displacement as far as I can tell so this calculator works.
Any suggestions on where to purchase a floscan?
fgarriso
Guru
It works for both, but does not take into account high speed running surfaces ( above 40knts)
High Wire
Guru
No but they are still in business
Floscan Instrument Co. Inc.
Floscan Instrument Co. Inc.
fgarriso
Guru
Comodave
Moderator Emeritus
- Joined
- Jul 2, 2015
- Messages
- 21,276
- Location
- Au Gres, MI
- Vessel Name
- Black Dog
- Vessel Make
- Formula 41PC
The new site says they have the transducers but they don’t have the gauges anymore.
Benthic2
Guru
Have you figured out how many hours you'll have to run to save enough gas to offset the cost of new props ???
To get a good measure of your fuel consumption at various speeds, watch how kayakers handle your wash. Whenever you increase the height of your wave, your fuel consumption takes a noticeable increase. At 7 knots v 8.7 there will be a significant difference. Your fuel savings by slowing that 1.7 knots will far outweigh anything you could accomplish by changing prop pitch or tweaking your engines.
Nomad Willy
Guru
rslifkin wrote;
“So keeping your current 23x26 size but going to a modern 4 blade design is likely in the ballpark.”
What on earth is old fashioned about three blade props???
The numer of blades on most any boat are best determined by numbers.
The concept of proper blade loading is boss. Yes it’s all a mater of propeller loading .. and that is best done at the engine’s rated rpm.
But when you’re shooting for efficiency the planning and adjustments to typical max power being the point that all the numbers revolve around is up for much guessing or even more computations.
But there are too many good reasons to prop for rated rpm. Cruise speed economy will be not optimal but probably fairly close.
And one of the best bits of information re # of blades is that the lowest # of blades gives the greatest efficiency. But few of us have the room to swing a two blade propeller most run 3 blade props.
Reasons to use a 4 blade revolve around blade area loading and diameter capability.
So the situation and configuration of your boat determine how many blades. There are some lesser variables like blade design but most of us will be running much the same style/design blades.
So is usually three blades unless you can’t get the blade area you need for the power you have on hand. Unless you’re overloading a three blade you won’t benefit from a 4 blade.
However when your numbers and computations are right between 3 and 4 blades you could pick for a far lesser variable. Most think 4 blades are smoother .. and they may be but probably not enough to choose over that. A better way would be to choose what seems best at cruise rpm. Or choose a prop that has a much better reverse thrust.
“So keeping your current 23x26 size but going to a modern 4 blade design is likely in the ballpark.”
What on earth is old fashioned about three blade props???
The numer of blades on most any boat are best determined by numbers.
The concept of proper blade loading is boss. Yes it’s all a mater of propeller loading .. and that is best done at the engine’s rated rpm.
But when you’re shooting for efficiency the planning and adjustments to typical max power being the point that all the numbers revolve around is up for much guessing or even more computations.
But there are too many good reasons to prop for rated rpm. Cruise speed economy will be not optimal but probably fairly close.
And one of the best bits of information re # of blades is that the lowest # of blades gives the greatest efficiency. But few of us have the room to swing a two blade propeller most run 3 blade props.
Reasons to use a 4 blade revolve around blade area loading and diameter capability.
So the situation and configuration of your boat determine how many blades. There are some lesser variables like blade design but most of us will be running much the same style/design blades.
So is usually three blades unless you can’t get the blade area you need for the power you have on hand. Unless you’re overloading a three blade you won’t benefit from a 4 blade.
However when your numbers and computations are right between 3 and 4 blades you could pick for a far lesser variable. Most think 4 blades are smoother .. and they may be but probably not enough to choose over that. A better way would be to choose what seems best at cruise rpm. Or choose a prop that has a much better reverse thrust.
Last edited:
FF
Guru
- Joined
- Oct 12, 2007
- Messages
- 22,552
"My general cruising speed is hull speed so 8.7 knots and it takes 2,000 RPM to get there. Any suggestions?"
I assume you have a 4bbl carb.
2000 RPM at hull speed will require less at cruising speed .
AS an experiment I would disconnect the large secondaries to see if the boat will run at cruising speed with out opening the "defulers".
If it does fine , an adapter plate and a modern 2bbl carb might be a simple solution for your use.
I assume you have a 4bbl carb.
2000 RPM at hull speed will require less at cruising speed .
AS an experiment I would disconnect the large secondaries to see if the boat will run at cruising speed with out opening the "defulers".
If it does fine , an adapter plate and a modern 2bbl carb might be a simple solution for your use.
rslifkin
Guru
- Joined
- Aug 20, 2019
- Messages
- 7,583
- Location
- USA
- Vessel Name
- Hour Glass
- Vessel Make
- Chris Craft 381 Catalina
I was thinking 2 different things. Newer props with a more modern blade profile. And also maybe more blade area, as he's bordering on under-propped, so a hair more pitch or blade area might not hurt in the performance department.
I wouldn't bother going to a 2bbl. The primaries on a quadrajet are already tiny and meter fuel fairly well, so a 2bbl is unlikely to be much (if any) better under light loads. And you'd lose the ability to throttle up if ever desired.
FF
Guru
- Joined
- Oct 12, 2007
- Messages
- 22,552
An even lower cost solution might be to install a vacuum gauge.
As you advance the throttle there will be an RPM where the vacuum drops a large amount. That is the secondaries opening .
Stay with the high vacuum reading and see if the boat is up to cruse (not hull) speed.
As you advance the throttle there will be an RPM where the vacuum drops a large amount. That is the secondaries opening .
Stay with the high vacuum reading and see if the boat is up to cruse (not hull) speed.
