Repower A25 with Izusu

[Fredstraw1]

Hello. I am looking at repowering my A25 with a 27hp Izusu. The engine has a gear with a 1:1 ratio. For those members with a similar engine, what gear ratios are you running? This replacement engine does have a prop but I have no idea if our shaft tapers are compatable.

[WillieC]

The original shaft was likely 30mm with 1/10 taper. 30 mm to 22.5 diameter over 75mm of shaft length. With a key.

[DesertAlbin736]

From your photos it appears Mystic is an early model pre-deLuxe full displacement hull version. In which case 27 HP is about right. Our 1971 model was repowered with 24 HP (27 HP/3600 RPM for limited full throttle duration) Yanmar 3GM30F back in early 1990s by previous owners (previous to the owner we purchased from). Our gearbox ratio is 2.61:1 turning a 16.5 X 12 pitch prop. Normal cruising for us is 2,350 engine RPM at 6.5 knots. With our gearbox the prop RPM is 900 at 6.5 knots. Can't help with shaft diameter and prop issues with 1:1 gear ratios or what pitch & diameter prop would match that configuration.

[A251975]

If you have the early A25, I would expect a 25 mm dia. Prop shaft with an ISO 10:1 taper. The only 27hp Isuzu marine engine that my limited search found is a Pisces which is a 2 cyl. with 27 hp @ 2800rpm. Many of them came with a Borg Warner 70C trans with possible ratios of 2.1, 2.57, 2.9, & 1.9:1. A 1:1 trans ratio seems unusual to me. There was an engine dealer in Seattle (Ballard) that converted Isuzu engines for marine use. An exact knowledge of the engine hp and rpm specs and trans ratio is needed before any prop decisions. Also, your style of cruising is helpful, eg. I have a large 4 blade prop that gives me a smoother and quieter cruise and really powers thru the waves. However, most prop experts will criticize it as ‘overpropped’.

[tribologist]

I have an industrial Isuzu 3CB1 with a 2.1:1 ZF10 transmission in our classic from -71. On a non delux you have no use for more power. I typically let is run around 2100 rpm. Top is about 2750 rpm. Pesk power is at 3200 so it is overpropped. 1:1 gear ratio seems strange. Call a prop place like Michigan propellers and hear with thrm

[WillieC]

Agree. Sizing the prop is a simple matter for folks that size props all day long. All they need is correct information about the vessel.