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I'll be posting pics and exactly what has unfolded over the last month. But, engines are in and tested today. The prop shop suggested 4 blade 19x20 nibral props. We ran her at 31 knots but at only 3300 rpms. Engines rated to 3800 so have some work to do on the props. Gear ratio is 1.5:1...not sure why they are so far off. Props off and back to shop...should have an update on Friday.

For a 3800 RPM diesel, I would think:

2:1 (though I always wanted to try 1.85:1)

21X23

What Mike said.

The guys at Marine Parts Supply in Canada hat the 1:58 gears also, but 3 blade 19x18 prop. Their test showed 30 knots at 3860. May have to switch props. What would be the advantages of going with the 2:1 gears?

2:1` gear reduction should cut the RPM's from the engine to the prop in roughly half. At 1:1 you have no reduction. So an engine turning at 3000RPM's turns the prop at the same speed.

At 2:1 an engine at 3000 rpm turns the prop at 1500 RPM's. This should allow you to spin a slightly larger wheel.

I have Cummins 270's that are supposed to top out at 2600 RPM's. My transmission is 1.5:1 so I am turning at 1,733 and I can spin either a 20X23 4 blade or a 21X20 four blade with roughly the same results. Difference is that the drop in pitch is making up for the increase in diameter. I also have cup added to my 20X23 prop which is equivalent to adding pitch but it is more efficient because it helps reduce slippage.

If you want to get more interesting you can read Bob Lilco's posts on Props. What he is doing is saying what size blank is a 21 inch prop cut from. Lets say they cut a 2 inch prop from a 23 Inch blank. He is taking the blank the next size up, which for argument sake might be a 26 inch blank. They cut that one down to a 21 Inch diameter. That gives him more blade area, making the prop in theory more efficient. More blade area more water to push. Believe it our not does not work on every hull.

I know someone who did the same thing on his 38 Henriques and he picked up 2 knots at cruise and cut his fuel burn. But the question is, is that worth the extra money to play with the props that way. You really have to run the boat a lot.

With more reduction you can turn a larger wheel.

More diameter = less slippage, more efficient.


To a point, then its becomes too much drag and larger works against you.


Then you have the wheel clearance issue...more wheel diameter "can" mean more shaft angle to keep proper clearance.

More shaft angle decreases efficiency as thrust is less inline with direction of boats travel.
Water gets pushed downward instead of straight back...that's why outboards, I/O's and Pods are more efficient then inboards.


Or in short, thanks for the update, let us know how the new wheels work out for you.
I have my fingers crossed for you!

Tony Meola wrote:2:1` gear reduction should cut the RPM's from the engine to the prop in roughly half. At 1:1 you have no reduction. So an engine turning at 3000RPM's turns the prop at the same speed.

At 2:1 an engine at 3000 rpm turns the prop at 1500 RPM's. This should allow you to spin a slightly larger wheel.

I have Cummins 270's that are supposed to top out at 2600 RPM's. My transmission is 1.5:1 so I am turning at 1,733 and I can spin either a 20X23 4 blade or a 21X20 four blade with roughly the same results. Difference is that the drop in pitch is making up for the increase in diameter. I also have cup added to my 20X23 prop which is equivalent to adding pitch but it is more efficient because it helps reduce slippage.

If you want to get more interesting you can read Bob Lilco's posts on Props. What he is doing is saying what size blank is a 21 inch prop cut from. Lets say they cut a 2 inch prop from a 23 Inch blank. He is taking the blank the next size up, which for argument sake might be a 26 inch blank. They cut that one down to a 21 Inch diameter. That gives him more blade area, making the prop in theory more efficient. More blade area more water to push. Believe it our not does not work on every hull.

I know someone who did the same thing on his 38 Henriques and he picked up 2 knots at cruise and cut his fuel burn. But the question is, is that worth the extra money to play with the props that way. You really have to run the boat a lot.

Tony do you have quick access to Bob's posts on this topic? i swear ever time i search i get the same stuff and its not there

Mizerita wrote:What would be the advantages of going with the 2:1 gears?

Being able to take advantage of the low rpm torque of the diesel, something the gas engine simply doesn't have (with proper marine cams).

The efficiency is nice, the lower possibility of explosion is nice. But the really great thing about the diesel is the difference in performance under 2000 rpm. When you're trying to run the boat up hill into a snotty inlet at 8 knots, you'll get the picture. But you'll want 20" four blade or 21" three blade props. 20X21, 20X22, 21X21, 21X22, even 21X23. You need 2:1 gears to turn those guys both fast and slow enough. 18 and 19 inch props are for gas engines. And a 21X22 blade with 1.5:1 gears won't be able to go slow enough to safely maneuver around the dock.....

2nd run today with props "tweaked" a bit...just under 30 knots but ran 3900 so a 100 over the recommended RPM. Boat was somewhat light, but I still may add some cup and go at it again. Hoping to get back to 30 knots.

I would also like to find Bob's prop post...if anyone can send a link I'd greatly appreciate it. Boat should be back in Key West next week and will send final pics and what was done.

Mizerita wrote:2nd run today with props "tweaked" a bit...just under 30 knots but ran 3900 so a 100 over the recommended RPM.

Tweaked how? How can you run 31 knots at 3300 rpm and 30 at 3900? Did you go to a 16" prop? I fear that you're making a mistake.....

bertram31.com/newbb/viewtopic.php?f=2&t=12586&p=116235&hilit=props#p116235
http://bertram31.com/newbb/viewtopic.ph ... ps#p107868
http://bertram31.com/newbb/viewtopic.ph ... ps#p105067
http://bertram31.com/newbb/viewtopic.ph ... ps#p102697
http://bertram31.com/newbb/viewtopic.ph ... ps#p102589

Unfortunately it is not in one posting. To get it all you have to go thru a couple of posts. Here are 5 that might help. You really need to read them from top to bottom to get everyones comments and to get an understanding of what Bob has done.

Mike, the prop guys suggested 19x20 4 blade Nibrals. When they ran it the first time they said 31 knots at 3300. They changed the pitch and ran it again and said 3900 at 29.5 knots. The boat will be back here in Key West this week and plan to run her full of fuel, ice, and people and see what the numbers look like then change accordingly. I wouldn't have thought they could have changed the pitch that much either. I'll know more this week when I can run her myself.

mike ohlstein wrote:
Tweaked how? How can you run 31 knots at 3300 rpm and 30 at 3900? Did you go to a 16" prop? I fear that you're making a mistake.....

19 x 18 or 19 x 17 ?
Pulled out some cup or tried 3 blade

So he changed the pitch from 20 to 16.5. I'll run her once the wind lays down by Sunday or Monday here in Key West. I'm not confident on their speeds as they didn't pay attention to wind, current, etc. Latest speed with Nibral 19x16.5 4-blade were 3900 RPM at 29.5 knots.

29.5 will probably be about it for you on the top end. You may be able to squeak it up to 30 or 31 but you have about hit it.

I'm guessing that you are under loading the engines.

Those gears are a mistake, and trying to correct that mistake by using the wrong props is only making things worse.

mike ohlstein wrote:I'm guessing that you are under loading the engines.

Those gears are a mistake, and trying to correct that mistake by using the wrong props is only making things worse.

Mike, I'm not arguing with what you are saying...but if those are his numbers he can add some cup increasing top end speed while still having a slow idle speed. Maybe not ideal, but why is this such a mistake.

I say if as numbers seem a bit unreliable of, could be going from over wheeled to. Under with such a large pitch change.

I don't think that he's going to be able to get out of his own way at low speed, and won't be able to take advantage of the low end torque when slogging home through 8 footers at 11 knots. It's simply not going to handle like a diesel boat. It defeats the purpose of being able to turn a large wheel.

You may not remember, but about 15 years ago we had a member who had overbuilt race engines that would turn 5000 rpm all day in his boat. He had 16X16 wheels and direct drive transmissions. I was on his boat. It did 50 knots, but took about 55 seconds to get up on plane. Fine on a glassy day in the bay, but for heavens sake, stay out of the inlet on a snotty day. It also had a range of about 80 miles on 200 gallons of 111 octane gas. Any way you slice it, there's a huge difference between the way that this particular boat reacts to a 19X16 wheel and a 21X22 wheel. Huge. Really huge.

There's at least 50 years and eight million boat/hours of experience behind this way of thinking. This is an expensive project. I hope it works out, however that is.

Mike- I thought I remember someone building high performance small blocks, don't recall the outcome. 5k running had to have to torque curve real high much beyond what is considered normal.

I am just thinking of my boat with gas 440's. I had 1:1 with 15x11 wheels. In flat to light snot she popped on plane as fast as I'd dare push the throttles. Used as a ski boat once and only once as it was a real expensive ski boat for lousy skiers always needed to start and stop...then tubing...just fast. Went through a full tank of fuel in a couple hours.

Climbing the back of swells and big waves the issue was the motors not being powerful enough to maintain RPM's without adding throttle, then having to cut back after going over.
I am not sure I noted much difference when I changed the gears to 1:1-1/2 with a larger wheel, aside from economy and less vibration. My thought is its same motors with same power curve.

Guess I'll see in the spring if it's such a dramatic difference...although they are low hp...but with lots of torque. I've always seen it as rearranging the numbers with somewhat silimalr outcomes based on where you want your best performance.
If. Your looking for top speed your going to sacrifice low end torque. Want low end torque, don't expect a speed demon.

If you want both you need to up the initial power of motors. Like me, 200 hp, I' expect the low rpm torque with very limited speed as motors poop out soon after Im going to be on plane.


I've always seen boats as a compromise...you have to compromise between torque, speed and money.

Carl

The difference between diesels and big blocks is night and day. Even at 210 HP you are going to notice a big difference. No lag trying to get over the hump when climbing a wave. Instant response to the throttle. Just night and day. It is all because of the torque curve and the bigger wheel.

The only thing is that maybe were we are off base on these Hyundai's is that they are more like big blocks spinning higher RPM's more like the Yanmars. How big a wheel is Dug spinning with his 240 Yanmars?

I have the 230 Yanmars and spin a cupped 21X22.

mike ohlstein wrote:I have the 230 Yanmars and spin a cupped 21X22.

Mike, what is your cruising speed and at what rpm? Same thing with WOT?

My throttle cables are a little stretched out, so until I adjust or replace them the best I can do is 3300 rpm (even in neutral).

That said, I cruise at 23 knots at 3100 and currently top out at 24.5 at 3300.

mike ohlstein wrote:I'm guessing that you are under loading the engines.

Those gears are a mistake, and trying to correct that mistake by using the wrong props is only making things worse.

I've run the boat twice with similar performance. Last week with 4 adults and 3 kids, ice, 220 gallons of fuel, etc...WOT at 3950 rpm (150 over max rating) and 28.5 knots. 3300 rpms got me 25 knots at 19 gph total. This weekend we had 5 adults, relatively full and a nice 2-3 foot chop with consistent winds out of the SE. When we could, we cruised around 3100 rpm, 18 gph and 23-24 knots...but was too rough to get WOT. She ran great and amazingly dry to and from Boca Grande Key. Speed around the docks, depending on tide, was anywhere from 4.5mph to 5.5mph with both engines...but very responsive and much easier to dock.

The prop guys gave me the final prop reports - measured diameter 19.93 and pitch measured at 16.4. Mike, there are at least 2 other guys with B31s running the 1.58 gears, but 3 blade props. I'm guessing I can add a little cup as we ran heavy both times and still 150 over. My only concern is does it hurt the integrity of the prop by changing the pitch as much as they did?

I'm not overpropped (most important), speed and cruise numbers seem normal, speed at idle /dock is fine, boat seems to run well balanced and dry...am I missing something?

I can't argue with the numbers, they seem right.

Are you sure that they bent your wheels and didn't swap them for others?

You can only add or remove 2 degrees of pitch on propellers. For example if they are 19 pitch, you can make them 17 or 21 pitch.

They may have. I think originally they thought I was running 2:1 gears and when I called them out on being so far off, maybe they switched them...In any case, I don't think I'm too far off. I'll keep everyone posted on the performance over time. So far so good... smooth, responsive, and quiet.

Glad is worked out well. A few pics of the install, and maybe a video of her running would be really nice....

Not familiar with those engines but the only thing I would have expected would have been the GPH at cruise to be closer to 15 gals per hour rather than 18. But compared to big blocks, 24 knots at 18 GPH is great.

I have been trying to figure out which of the two, the 28 or the 31, are more economical in their fuel consumption, by comparing various data found on various threads.
The difficulty lies in that one has NOT all the elements at their disposal (weights, weather, bottom condition, enclosures etc) so as to arrive to safe conclusions.
Let alone that the comparison of diesel boats with identical engines is hard to come about.

My 28 with Yanmar 240's burns around 60 liters per hour at 23-24 knots.
So the 270's which are still tight that burn the same or slightly more seems OK to me.

In my case, running at 18 knots instead of 23-24 does not present any measurable advantage in economy.

Yannis

There is a difference in hull configuration between the 28 and the 31. So that plays into the fuel consumption between the two boats. Plus, for some reason when you stretch a boat a foot or 3 sometimes they actually perform better.

Thanks Tony.

All this is theoretical, of course, as any difference in fuel burn between the 28 and the 31 is so small that it cannot by itself be a decisive factor to choose one over the other.
Also, after the tons of work I have put into my own boat, and although I may sometimes wish I had a slightly bigger one (not a 31 but a 33 with pleasure), I stop thinking about the issue immediately, as I cannot see me start the same cr@p all over again, let alone the costs. The law of diminishing marginal utility...

John F. wrote:Glad is worked out well. A few pics of the install, and maybe a video of her running would be really nice....

I'll post some pics of the install and updated numbers once I get back from vacation. Definitely a few hiccups, but overall I'm pleased.

Tony Meola wrote:Not familiar with those engines but the only thing I would have expected would have been the GPH at cruise to be closer to 15 gals per hour rather than 18. But compared to big blocks, 24 knots at 18 GPH is great.

I was hoping for a little better, but both times we ran her it was blowing 15-20 knots and a nice Key West chop...we also had full fuel and 5-7 people on board. Hoping to get better numbers to/from the reef on a smooth day and maybe a little tweaking of the props...but quite frankly after getting .75 mpg at cruise with my gas engines, I'm content.