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Old 19-12-2020, 20:51   #61
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Re: Swapping Perkins 4108 to Beta 50

+1for the beta 35.

I also had a 4.108 and have been through repowers with both yanmar and the beta. Beta is superior in both operation and maintanence.
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Old 19-12-2020, 21:34   #62
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Re: Swapping Perkins 4108 to Beta 50

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Originally Posted by Cpt.Steve View Post
** For others who have gone from a 4108 to a modern 50hp. Did you have to go up in prop shaft size? What did your reps suggest? Our new boat was soecd with a 1 shaft for a 4107/8 with 2:1 reduction (BW vd to a Walter V). Curious if anyone is running a 1 with 45-50+ hp. Thanks
When I replaced my 4-108 with a BV2203 (Beta 50) the factory said my 1-1/4" shaft was marginal. I kept it and it was fine. But 1" is too small. I'll bet you never drove that 4-108 to full power. Your prop is probably undersized for that engine. If you were able to get the 4-108 close to 4000 rpm you were never really loading it up, I couldn't get it up to 2200 so my installation was the opposite.

If you were never using all the power of the 4-108 you don't need a Beta 50. The Beta factory suggested I only needed a Beta 43 to replace the 4-108 but my boat (44 feet 12 tons) was underpowered.
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Old 19-12-2020, 23:32   #63
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Re: Swapping Perkins 4108 to Beta 50

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Originally Posted by Cpt.Steve View Post
Scrub cutter, are you still pondering this repower? If so, I would implore you to reconsider the sizing, though I am sure beta would have told you the same.

An old 4108 would actually have used a different formula for determining HP. Compared to the Beta numbers its actually producing closer to 40hp, or even 37hp is often touted. We have a Beta 35 in our Tartan 37 at 16,000lb mostly dry and happy cruise at 6.5 pushing 7.2 when we really put it to her. I would think the Beta 38 would be a much better fit.


** For others who have gone from a 4108 to a modern 50hp. Did you have to go up in prop shaft size? What did your reps suggest? Our new boat was soecd with a 1 shaft for a 4107/8 with 2:1 reduction (BW vd to a Walter V). Curious if anyone is running a 1 with 45-50+ hp. Thanks
Shaft size is a function of torque, not necessarily horsepower. The gear reduction ration makes a huge difference.
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Old 20-12-2020, 03:53   #64
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Re: Swapping Perkins 4108 to Beta 50

A 4 108 is now an oldish engine 56hp at 4000rpm ,dont worry about hp,rpm and torque match these two to repower plus remember that variable gear ratios are available for most g boxes ,try to stay under 1200 shaft rpm if possible in a displacement hull ,shaft size must also be considered plus prop tip clearance . Have fun ,stay safe .⚓️⛵️
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Old 28-01-2021, 13:18   #65
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Re: Swapping Perkins 4108 to Beta 50

Sorry to all those who have put in some great replies on this thread and thanks so much for your wise thoughts. I have been doing a bit more research and have settled on the Beta 38 with the PRM125 mechanical box after confirming the current prop is indeed a 3 blade 16x13 (I will look at upgrading to a max prop in the future if this combo doesn't work) . Original box was (is) a Hurth 2:1 but recommendation was to go to 2.5:1 to match Beta 38 specs. Next challenge will be the mounts as the CD36 has cross bars which form a box section for the feet. I have around a 4 month wait for the engine so will have some time to come up with a solution. Unfortunately Cape Dorys are a rarity in NZ, although there is a CD30 in my marina.
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Old 28-01-2021, 15:18   #66
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Re: Swapping Perkins 4108 to Beta 50

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Originally Posted by Wainui View Post
The rear main crank journal is scored so it keeps leaking oil.
Needs a new crank!
Im planning on changing this 52 year old engine out to a Betamarine 50 hp.
I have a Velvet Drive transmission which has just had the seals and o rings replaced. The clutch packs were in great condition.
Does it make any sense keeping the old transmission to save money or should I get the BM transmission?

26000 lb boat
Walters Vdrive
17 Max prop 3 blade

Many thanks
The Perkins 4.108M was factory derated to 3000 rpm, where it made 40hp, when installed in a displacement boat. The Beta 38 would duplicate the Perkins power, while being 300 pounds lighter. A Velvet Drive mounting kit is also available for the Beta 38. 50hp is not recommended for a 26k pound boat unless you regularily fight waves, strong currents or high winds. One BIG limiting factor is the prop. What is the maximum diameter prop that will fit your boat?
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Old 28-01-2021, 18:43   #67
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Re: Swapping Perkins 4108 to Beta 50

Hi Key
Have to disagree about the hp required. Most boat manufacturers calculate 1hp per 500lbs. The boat was a little underpowered in a seaway and wind. It came with a 18” x 2 blade prop. I now have a 17” x 3 blade Max Prop.
I can now motor at 6.5 knt @ 2200rpm with plenty to spare.
Love the Beta!
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