We currently have a 2007 315 and are considering changing up to a 348. I'd be interested to hear from anyone else who has made the same change but one question we have right now is on the engine. Some sources seem to say the 29hp engine is the favoured option? Since our main use is coastal day sailing mainly south coast of UK, I just wonder whether the larger engine would have many advantages for us?

Any thoughts welcome

Thanks 

Colin 

Having changed the MD2020 volvo in my 2003 version 311 for a D1-2030 & new saildrive, I find the difference is well worth it. That is on a 31. I could not imagine a 20 Hp on a 348 being much good.

My performance (With the same Brunton Autoprop) has gone from cruising average 5.9Kts & max in flat water and no wind 6.8 kts, -- to cruising 6.5kts & max out at 7.5 kts in a bit of chop & quite breezy head wind. Drive through 1metre chop has improved considerably, which is a problem with Brunton Autoprops.

Of course this a 31 with an autoprop & does not fully answer the Op's question but I would really consider the larger power unit.

Speeds are through water irrespective of current.

Is more horsepower better? Weigh out the differences in weight, dependability and fuel economy. In prior power boat progression, we went from a Volvo Penta D3 @ 200 HP on a 27' cruiser, to a D4 @ 300 HP on a 31'. Fuel economy suffered substantially, weighed against a more spacious and seaworthy boat.  

I would make the point that on both my single handed round UK trips (Via the calledonian canal) I used the engine 30% of the trips. Others have reported just as high % some more.

5.7Km/h is just slightly above walking pace & if one had to meet a tidal gate against a head wind & chop that would be reduced significantly. 

Trying to cover the last 3-4 miles in to Dover having missed the tide, against wind & a steep chop can be hard work. Sailing is not always the option.

As for fuel consumption I consider the cost of a couple of extra litres of fuel irrelevant in the grand scheme of things

The 30 is 20KG more than a 20 which is not that much to affect a 35  ft boat low down so just put a little less excess baggage on board when going off on a trip

nm/hr at crusing rpm ( ~ 2000 - 2200 rpm ). 

Average Consumption is 2 ltrs / hr on ny d1- 30 measured over 500 motor hours. On my last boat (30") I had a 21 hp Yanmar and got an average og 1.8 ltr/hr.

I have just come back from a 21 day holiday in Ostend so i took a much closer look at the figures. This time I had my boat lifted & jetwashed on the 7Th august. I made sure that they also washed under the straps as well . We departed on the 10th so the hull was perfectly clean

I had filled the tank & had run the engine in the berth for 2 hours charging batteries on a fast tickover. We left for Ostend & motored the 88 mile trip (Due to my health)  taking exactly 13 hours berth to berth. We ran at 2300 revs the whole way It was quite rough & we got a good soaking, with beam winds of 5-6. When we arrived we had to change berths through the lock etc so had another hour of tickover or light running & finding a berth

That totals 16 hours. We needed exactly 41 litres to top the tank again.

On the way back we went via Ramsgate -58 miles -then to Bradwell the following day I did not log the fuel but decided to hamer it and winds went from 18-30 kts at times we were travelling at over 7.4 kts TRUE speed The GPS recording 8-9 kts in places. I was surprised what speed could be produced & the bow wave was really impressive. 

So a Volvo D1 30HP engine will greatly improve the power. A 311 WILL reach hull speed But one WILL burn fuel. But It is there when you need it & on the trips there & back we did .My Volvo MD2020averaged  over 2 litres per hour over 5000 hours. It would have been geat sailing weather but I could not have been capable. Pity as it is my last trip before selling after 22 years