A cute little motor-sailer!
The two-part mast not unusual these days. I’ve done it (but with two components of aluminium tube – I’ve also done a wooden extension at the heel). It wasn’t difficult. I avoided any mechanical fastenings and just used epoxy for the join – it seems OK but I am persuaded now that a slightly flexible polyurethane rubber “goop” might be better, especially when bonding two dissimilar materials such as wood and aluminium. Other people who know more than me will hopefully chime in here.
The transition between the two components needs to be faired so the transition from one diameter to the other is smooth. That’s no problem with wood – and you have the chance to make the top part tapered too, which is good.
I really wanted to comment on your concern about strength. People mostly worry about the bending strength, but if there is sufficient overlap (sufficient bury of the wood into the aluminium pole) that won’t be an issue, and I don’t see the need for any mechanical fastenings (and drilling holes should be avoided anyway).
The issues that people tend to overlook (at least, in conversations I have had) are two fold: (1) There will be a downward force on the top component, due to the downward pull of the halyard. The fairing which brings the diameter of the top component into line with the lower component means there will probably be a lip at the join and this should be enough to resist that force and prevent the mast wanting to “telescope”. And (2) (surprisingly perhaps) a rotational force on the top component due to the wringing (twisting) effect of the halyard spans as the sail is sheeted in and out. The entire mast has to be fixed against rotational forces, and the top part of the mast must be able to resist rotation with respect to the lower part.
I think polyurethane rubber glue (such as Simsons which comes in a tube) together with the lip at the point of transition will be enough to take care of all the forces, provided there is enough “bury” and personally I would prefer not to drill holes or add mechanical fastenings. I would be keen to know what others think, as quite a few hybrid masts have been made and all pretty well tested now I should think.
How much bury is needed? That’s the only real question, I think. I used the old 10% rule but it kind of didn’t seem enough so I added a bit more for good measure. But of course, you don’t want to be adding any more weight than necessary up there.
I look too forward to reading some advice on that question.