Repower Regrets?

[kenbo]

Originally Posted by limte

Except for the hp the specs for the Beta 35 and Beta 38 are identical. Same displacement, same weight, same engine dimensions. The 38 is the same engine as the 35, but tricked out to produce more hp at higher rpms. The Beta 35 produces 35hp at 2,800 rpms; the Beta 38 produces 38hp at 3,600 rpms. The TMC 60 gearbox is recommended for both engines. The cruising rpm for the Beta 38 is 3,000.

That "trick" applies to most diesels. Some call it continuous rating vs. intermittent.

I don't know if this is widely known to marine diesel owners, but you can measurably increase the shaft HP by advancing the timing. Many engine manufacturers retard the timing to help meet emissions tests. The gas-to-diesel swap 4x4 crowd has been doing this for years. They mostly use Kubota 4-cylinder engines but it's easy to do with any cam-driven FI pump using thinner or less shims under the pump body.

[bobmcd625]

Clearly it's a very personal choice but for my 2 cents I'll share that the original motor specified by the builder for the DE38 was 32 hp. I have a 1975 DE38 which, when I bought her in 2016, had a Volvo 2000 18.8 hp engine which was installed by the original owner in 1991. That engine had serious problems so I replaced it with a Volvo DE3-40 with which I am totally happy. To be honest there was no debate as you are doing now about weight vs. speed etc, but we cruise happily at 6 - 7 kn at 2500 rpm. The Volvo people say these engines love going for long periods at high revs but I never push her to the limit. I am sure you'll be happy with 35 hp but it's totally up to you.

[moosemaxmann]

Originally Posted by bobmcd625

Clearly it's a very personal choice but for my 2 cents I'll share that the original motor specified by the builder for the DE38 was 32 hp. I have a 1975 DE38 which, when I bought her in 2016, had a Volvo 2000 18.8 hp engine which was installed by the original owner in 1991. That engine had serious problems so I replaced it with a Volvo DE3-40 with which I am totally happy. To be honest there was no debate as you are doing now about weight vs. speed etc, but we cruise happily at 6 - 7 kn at 2500 rpm. The Volvo people say these engines love going for long periods at high revs but I never push her to the limit. I am sure you'll be happy with 35 hp but it's totally up to you.

18hp?! Docking must have been a hoot. Great to hear you can hit 7kts in the DE38 under power. What did you decide to go for with the prop? Thanks for the advice. Great to meet another DE38 owner in here.

[Icarus]

Originally Posted by moosemaxmann

Overall Question: Do I need to modify my boat or my mindset?

We are in the process of refitting a multigenerational family-owned Downeaster 38. Alas, we are at the repower phase. I'm agonizing over the Beta 35 vs. 43. The Beta 38 would need at 3200rpm to cruise vs. the lower-revving alts...meh

As a prior race boat captain, 6.5kts seems like the minimum cruise speed I wanted for deliveries, particularly in unfavorable water when "get-there-itus" set in.

With the DE38's 7.2kt theoretical hull speed, I could get a 6.5kt cruise out of the 43, but it would require boat modification to swing the necessary 8" prop. The direct plug and play Beta 35 would get me to a 6kt cruise/6.5kt WOT. It would be better in every measurable way over the existing and constantly overheating Universal 5432. The dealer is pushing for the 35.

Has anyone repowered and regretted not getting a higher power range? Should I listen to the experts, and chill myself to accept 6kts or listen to my FOMO and go for the bigger option. Kindly endow me with your $0.02

It comes down to gearbox ratio, RPM, max torque and prop diameter.
A 35hp is adequate
I have a 40hp 3 cyl ISUZU, max torque at 2000 RPM at 30hp...prop dia 18"×12" cruising speed 6kn at 1800 RPM, gearbox ratio 2.1:1, fuel consumption about 3 litre/hr
The boat is a 40' ketch, 12t displacement, 36' WL., so max hull speed is about 7kn.
A more powerful engine would do little to increase speed but would increase fuel consumption.
I would stick with a lowe rev 35hp diesel and at least a 2:1 or a 2.5:1 gbox if you want to swing a more efficient larger prop.

[Shaneesprit]

Originally Posted by hpeer

Well I have a 16,000lb 33’ boat with a 13hp engine. It gets me around, I have motor sailed and motored a few thousand miles. So I guess I am not the right guy to ask.

Sounds perfect 👍

[FPNC]

Your vessel has an aperture that will limit propeller diameter. Look at the maximum propeller size that will fit the aperture (with proper tip clearance) and calculate how much HP that a propeller of that diameter can efficiently transfer to the water.

Displacement boats are about efficiency. There is no reason to buy and install more HP than your propeller can efficiently transmit to thrust in the water.

Many Down East 38's were fit with 16" diameter propellers. If you are limited to a 16" diameter, 3 blade propeller, the 38hp motor will max out the blade loading of the propeller. No reason to go to 43 hp with a 16" diameter propeller as you will end up with excessive pitch and the efficiency will be decreased. Typically, the minimum diameter for a 43 hp motor would be 17". That extra 1" diameter creates a lot of extra blade area. 18" diameter would be better.

If you can fit a 17" or 18" prop then the 43 hp engine makes more sense. If required, can the aperture be increased? Regardless, with a 29' waterline I would not expect much more max speed than what the 38 would provide.

You can make any combination work. Whether it works efficiently is another matter.

[BlueH2Obound]

The amount of HP desired for a sailboat depends a lot on whether you have a true sailboat, and use it to . . . well, . . . . SAIL . . . as opposed to having a POWER BOAT that has a big stick. Here, in the PNW, most of the "sailboat" owners apparently don't even know what those sticks and the big white things are really for . . .

[malbert73]

Originally Posted by BlueH2Obound

The amount of HP desired for a sailboat depends a lot on whether you have a true sailboat, and use it to . . . well, . . . . SAIL . . . as opposed to having a POWER BOAT that has a big stick. Here, in the PNW, most of the "sailboat" owners apparently don't even know what those sticks and the big white things are really for . . .

Sure.
Or, can be a pragmatic way to have more cruising flexibility- which is how I look at my performance cruiser also having an oversized engine.

[SV Confianza]

Originally Posted by moosemaxmann

Out of curiosity, what is your RPM range at cruise? My only hesitation going in that direction is having to run at comparatively higher RPMs than the 35.

Apologies I missed your question.

We have found that our best compromise between fuel and speed is around 2600 rpm. Boat is a catamaran, about 35,000 pounds, with two beta 38s, and we'll do about 7kts at that speed. Pushing to 2800 (I believe the "proper" cruise speed) does around 7.3-7.4kts. Pushing to 3400 rpm will bring us up to 8.1 or 8.2.

It's a 3600 rpm engine, but with the alternator running it settles around 3400 at full throttle. If the alternator kicks off, get a little over 3500. I might be slightly overpitched on the max props, and I may reduce one notch on our next haul out. We used the pitch recommendations from max prop for the betas and the seaprop 60 sail drives that come with them.

Lower rpm would not be good for the alternators, since they don't cool as effectively. We have two Balmar XT170's (ordered from beta and preinstalled) regulated by Wakespeed ws500's for Lithiums Bats and alternator temperature regulation.

[rslifkin]

Originally Posted by SV Confianza

Apologies I missed your question.

We have found that our best compromise between fuel and speed is around 2600 rpm. Boat is a catamaran, about 35,000 pounds, with two beta 38s, and we'll do about 7kts at that speed. Pushing to 2800 (I believe the "proper" cruise speed) does around 7.3-7.4kts. Pushing to 3400 rpm will bring us up to 8.1 or 8.2.

It's a 3600 rpm engine, but with the alternator running it settles around 3400 at full throttle. If the alternator kicks off, get a little over 3500. I might be slightly overpitched on the max props, and I may reduce one notch on our next haul out. We used the pitch recommendations from max prop for the betas and the seaprop 60 sail drives that come with them.

Lower rpm would not be good for the alternators, since they don't cool as effectively. We have two Balmar XT170's (ordered from beta and preinstalled) regulated by Wakespeed ws500's for Lithiums Bats and alternator temperature regulation.

You're probably slightly over-pitched from what you're describing. As far as cruising RPM, engine manufacturers will typically recommend a max continuous RPM (where that's the fastest you could cruise at), but cruising at a lower RPM is typically no problem.

[Bowdrie]

Originally Posted by Icarus

It comes down to gearbox ratio, RPM, max torque and prop diameter.
A 35hp is adequate
I have a 40hp 3 cyl ISUZU, max torque at 2000 RPM at 30hp...prop dia 18"×12" cruising speed 6kn at 1800 RPM, gearbox ratio 2.1:1, fuel consumption about 3 litre/hr
The boat is a 40' ketch, 12t displacement, 36' WL., so max hull speed is about 7kn.
A more powerful engine would do little to increase speed but would increase fuel consumption.
I would stick with a lowe rev 35hp diesel and at least a 2:1 or a 2.5:1 gbox if you want to swing a more efficient larger prop.

I've got engine specs close to yours, and running the same prop, (blade area could be diff,) but with smaller/lighter boat the engine just loafs along, an easy 6k at ~1550>1600.
I would easily give away 5hp to have a smaller and lighter engine, (and more real estate gained).
Your WL at 36' should have a max hull speed of ~8Kts.

Originally Posted by FPNC

Your vessel has an aperture that will limit propeller diameter. Look at the maximum propeller size that will fit the aperture (with proper tip clearance) and calculate how much HP that a propeller of that diameter can efficiently transfer to the water.

Displacement boats are about efficiency. There is no reason to buy and install more HP than your propeller can efficiently transmit to thrust in the water.

Many Down East 38's were fit with 16" diameter propellers. If you are limited to a 16" diameter, 3 blade propeller, the 38hp motor will max out the blade loading of the propeller. No reason to go to 43 hp with a 16" diameter propeller as you will end up with excessive pitch and the efficiency will be decreased. Typically, the minimum diameter for a 43 hp motor would be 17". That extra 1" diameter creates a lot of extra blade area. 18" diameter would be better.

If you can fit a 17" or 18" prop then the 43 hp engine makes more sense. If required, can the aperture be increased? Regardless, with a 29' waterline I would not expect much more max speed than what the 38 would provide.

You can make any combination work. Whether it works efficiently is another matter.

It's true, aperture size has been a "thorn in the side" for a lot of boats of that era, and build type, (full keel/keel hung rudder).
But back in those days boaters didn't expect big engines with an efficient power train, and alternators that pulled a couple of HP were not given much thought.
Also, harbors/moorings/docks/marinas/anchorages, were not even close to being as crowded and close-coupled as today.
You had less power, you operated slower, and you had more room and time to "fiddle around" with the boat making approaches and departures, less power was needed.
Today you use more power in bursts for sharper more abrupt maneuvering and driving aux loads.
Some over-propping ain't all that bad, not at all.
Just because you can't get that last couple/three hundred revs means very little in the vast majority of conditions, a little extra pitch gives better mpg, but you gotta pull the revs back.
You run at a S/L ratio of ~1>1.1, for a boat at ~28>29 WL that's somewhat over 5kts.
No yacht designer wants a slow boat, high efficiency in power trains is not as highly valued as it is in power boats
I would like a controllable pitch prop, but alas it'll not to be.