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Old 09-27-2021, 01:30 PM   #1
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Prop size and type

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?
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Old 09-27-2021, 01:49 PM   #2
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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.
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Old 09-27-2021, 01:55 PM   #3
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They are the crusader ones that give 350. Out of curiosity, why is going at hull speed a terrible speed? would it not be the most efficient speed to run at without being at idle or on a plane?
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Old 09-27-2021, 02:05 PM   #4
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They are the crusader ones that give 350. Out of curiosity, why is going at hull speed a terrible speed? would it not be the most efficient speed to run at without being at idle or on a plane?
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.
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Old 09-27-2021, 02:14 PM   #5
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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.
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Old 09-27-2021, 02:14 PM   #6
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Yeah my LWL is something like 37'5". I have noticed that at idle my speed is somewhere close to 4ish kts and like up until the 8kt range the wake is relatively small.
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Old 09-27-2021, 03:04 PM   #7
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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.
Attached Files
File Type: pdf Prop graphs.pdf (55.5 KB, 4 views)
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Old 09-27-2021, 03:07 PM   #8
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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?
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Old 09-27-2021, 03:12 PM   #9
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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).
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Old 09-27-2021, 03:22 PM   #10
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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?
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.
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Old 09-27-2021, 03:24 PM   #11
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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.
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Old 09-27-2021, 03:32 PM   #12
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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 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.
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Old 09-27-2021, 03:50 PM   #13
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Quote:
Originally Posted by Blarg21 View Post
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?
https://vicprop.com/displacement_size_new.php
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Old 09-27-2021, 03:51 PM   #14
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I think he'd want the calculator for a planing hull, not displacement.
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Old 09-27-2021, 04:12 PM   #15
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I think he'd want the calculator for a planing hull, not displacement.
The boat is a semi-displacement as far as I can tell so this calculator works.
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Old 09-27-2021, 04:13 PM   #16
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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.
Any suggestions on where to purchase a floscan?
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Old 09-27-2021, 04:20 PM   #17
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I think he'd want the calculator for a planing hull, not displacement.

It works for both, but does not take into account high speed running surfaces ( above 40knts)
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Old 09-27-2021, 04:32 PM   #18
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No but they are still in business
Floscan Instrument Co. Inc.
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Old 09-27-2021, 04:37 PM   #19
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web site for the new "floscan co"

https://floscaninc.com/
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Old 09-27-2021, 05:32 PM   #20
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changing the number of blades on you props will probably do nothing to improve efficiency. i would say there is nothing wrong with your prop config.



Quote:
Originally Posted by Blarg21 View Post
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?
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